I thought so too. But the topic is interesting. Mathematicians especially brag that what they do is valuable in proportion to its uselessness. Personally, I like spending time in useless things. Recently I was learning APL which is a totally useless endeavor for me. But then I look at other things I do and I cannot say that they are useful either. Then there is curiosity. There is no end to things to be curious about. I’m curious about so many things that I could never reach the end of curiosness with one topic. Curiousness or temptation comes at first as a very strong motivation but the next day the curiousness stregth diminishes and a new and stronger curiousness appears and attracts me so I go towards it forgetting the thing I was curious about the day before. Curiousness is also a question. We are curious means we are searching the answer for a question we asked or was asked to us. Something we saw or heard can show itself to us as a question. Then we go looking for its answer. But all answers raise more questions then they answer. So there is an infinite chain of questions. Curiosity is infinite. In her book The Manipulated Man Esther Vilar  writes that general curiosity for curiosities sake is a male attribute; a woman is curious only about things that have direct effect on her. This seems like a more sane and intelligent world view. That must be the reason all examples given in original article are men.
People learning "useless things" are often found in between pride and defensiveness. Wealth & ability can impress people. But dedicating so much time for yourself is sort of selfish.
The problem is that learning "useless things," which is rooted in a form play, turns into something more--that is a self-centered approach of securing yourself in a social hierarchy.
Gone are the humanities, the social sciences, the histories, the ancient linguistics, all of which depending on who you are could consider to be play around, while the rest of us serious people work for a living building software.
It was all for social status and personal play reasons.
Investing in ideas that may pay off in the future is an excellent idea. But except for the cream of the crop, few people will actually have their useless knowledge turn into something useful.
I suspect many people push this point about potential returns because they want to "get paid to play" as well as participate in a system that allows them to gain social status. If they convince others to support them using some argument about useless knowledge being useful, then they can continue in this way.
My only point is that I wish more people were open about that. Yes, research pays off, but if you are in a position of being able to learn lots of "useless knowledge," you are already doing well in terms of health, intellect and status. Don't bemoan too much about your life.
What is actually meant is trying to understand useful phenomena at a higher abstract level, or asking significant questions, i.e. what is the largest number? that lead to deeper insights into reality.
(Tangentially, whether a particular piece of knowledge is eventually found to be "useful" or not doesn't seem to have much to do with the intention or motivation of the person pursuing it. Also, I don't know there's much social status that comes from being an academic mathematician these days: other than 1 or 2 of my elderly neighbors, no one seems to think it's a big deal, least of all my students and other academics.)
In fact, if you enjoy them too much, you might need to forego wealth AND "securing yourself in a social hierarchy".
I have lots of friends in the cultural sphere who enjoy "useless things" for the sake of it, and don't have inherited money or anything or the short. Some work casual blue collar jobs to support themselves -- and they'd still relish the "useless things" if they were unemployed and living of coupons (as they have done for some time). No some young 20-somethings either.
I also don't understand what "realize you are wealthy in time, energy and cognitive power being able to do that".
Is that supposed to be something people should feel privileged? Having "time, energy, and cognitive power"? You don't need any more of that to learn "useless things" than for any other endeavors. Not only because you can (and people do) opt for less money or even poverty to devote yourself to "useless things", but also because the average person can (and tons do) devote some of the 2+ hours people use on social media BS and TV every day for "useless things" (from poetry and philosophy to theoretical math)...
Social signaling and play aren't the only thing that learning useless skills can turn into.
When I started learning to program, it was useless because all I knew how to write were toy programs. But at some point I got good enough at writing toy programs that people could actually use my useful work.
Right now, the fact that I've spent the last couple of months re-learning calculus is totally useless to my careers as a CRUD programmer, IT admin dude, or musician. But I started the MITx 6.002 class on electrical engineering, and that math learning has helped immensely with the first part of that course...
In itself me knowing how to design electronics is useless. And I am profoundly grateful to be , as you correctly phrase it "wealthy in time, energy and cognitive power". I can learn just because I find it playful.
But eventually I have some product ideas that I'd like to build (just for my own fun and enjoyment). And that doesn't sound useless at all. It sounds fun and interesting (which have some utility in themselves) and it could be monetarily profitable given the depth of my domain knowledge.
Similarly, when I started learning to play upright bass, it was just useless play... I didn't play well enough to do anything with it. At this point, I've done it enough that it's useful to the musicians I gigged and recorded with in the last week.
Similarly, I have a pretty good domain knowledge with IT skills, and at some point I want to turn my attention to the various methods of working with machine learning / statistics / AI / whatever. When I start to do that, it will be useless because I won't know enough to do anything other than play with it. But maybe eventually I will have enough facility to do some useful things with it... I am assuming that when I'm in my late 60s and 70s that will be a very useful skill to have for helping with political activism.
So I somewhat agree with you. Without actually doing anything, all knowledge acquisition is useless. I have most of a PhD in English and I totally agree that there is a lot of social signaling that goes on in education.
But the possibility of that use for education doesn't imply that social climbing and play are the only uses for "useless" knowledge.
"The Flexner Report is a book-length study of medical education in the United States and Canada, written by Abraham Flexner and published in 1910 under the aegis of the Carnegie Foundation. Many aspects of the present-day American medical profession stem from the Flexner Report and its aftermath. ... In a very short time, medical colleges were all streamlined and homogenized (all the students were learning the same thing) ... Flexner sought to reduce the number of medical schools in the U.S. to 31, and to cut the annual number of medical graduates from 4,400 to 2,000. ... A repercussion of the Flexner Report, resulting from the closure or consolidation of university training, was reversion of American universities to male-only admittance programs to accommodate a smaller admission pool. Universities had begun opening and expanding female admissions as part of women's and co-educational facilities only in the mid-to-latter part of the 19th century with the founding of co-educational Oberlin College in 1833 and private colleges such as Vassar College and Pembroke College. ... Flexner viewed blacks as inferior and advocated closing all but two of the historically black medical schools. His opinions were followed and only Howard and Meharry were left open, while five other schools were closed. His perspective was that black doctors should only treat black patients and should serve roles subservient to white physicians. The closure of these schools and the fact that black students were not admitted to many medical schools in the US for 50 years after Flexner has contributed to the low numbers of American born physicians of color and the ramifications are still felt more than a century later. ...When Flexner researched his report, "modern" medicine faced vigorous competition from several quarters, including osteopathic medicine, chiropractic medicine, electrotherapy, eclectic medicine, naturopathy and homeopathy. Flexner clearly doubted the scientific validity of all forms of medicine other than that based on scientific research, deeming any approach to medicine that did not advocate the use of treatments such as vaccines to prevent and cure illness as tantamount to quackery and charlatanism. Medical schools that offered training in various disciplines including electromagnetic field therapy, phototherapy, eclectic medicine, physiomedicalism, naturopathy, and homeopathy, were told either to drop these courses from their curriculum or lose their accreditation and underwriting support. A few schools resisted for a time, but eventually all either complied with the Report or shut their doors. ..."
Key treatment modalities abandoned as a result included spending time in the sunshine (which we now know gives you essential vitamin D), an emphasis on good nutrition (which is not a "procedure" doctors can be trained in and bill for), and other aspects of having a happier life like humor and so on (e.g. what Dr. Andrew Weil or Patch Adams write about). Alternative medicine has spent a century fighting back on such topics.
Another consequence of that report is that the American Medical Association and the "MD" established a stranglehold on medical treatments. MDs became in short supply and could charge large amounts of money. So medical treatment of any kind became much less accessible for most people. And when you did have money to pay a doctor, they would invariably be a white male and not a woman or minority who you might be able to relate to more easily if you were the same.
Also ironically, the Bamberger family, who wanted to give back to the city of Newark in some way for the success for their department store there by creating a medical school and teaching hospital in Newark, NJ, were instead talked into funding the physical creation of the Institute for Advanced Study in Princeton. Some of that is mentioned here: https://www.ias.edu/flexner-legacy
One big irony about Abraham Flexner is that he was very interested in hands-on education. While Flexner's excellent recommendations for making K-12 education "hands on" were ignored, his recommendation for making medical education "hands on" in terms of learning specific procedures were adopted by the mainstream instead of learning to see the bigger picture of a unwell person's life -- like Patch Adams advocated for instead with the Gesundheit! Institute. https://en.wikipedia.org/wiki/Patch_Adams
I have some other comments on that theme here:
"The Flexner Report a century ago (1910) began a purging process of alternative medicine practitioners in the USA. It lead indirectly to people like Herbert Shelton for being persecuted and prosecuted decades later for telling people age-old wisdom that sunlight, whole foods, and occasional fasting (and avoidance of stuff like cigarettes) could cure or prevent most chronic disease, and could do it better than mainstream medicine at the time (something that modern medical science is grudgingly coming to admit). Herbert Shelton may not have had the whole truth, but he had part of a bigger older truth, and he was harmed by a mainstream medical-financial system by advocating for that truth from the past and from his own experience."
I also include there a section of the older version of that Wikipedia article since removed: ""The Report (also called Carnegie Foundation Bulletin Number Four), called on American medical schools to enact higher admission and graduation standards, and to adhere strictly to the protocols of mainstream science in their teaching and research. ... One of the consequences of Flexner's advocacy of university-based medical education was that medical education became much more expensive, putting such education out of reach of all but upper-class white males. The small "proprietary" schools Flexner condemned, which were contended to be have been based in generations-old folk traditions rather than relatively recent western science, did admit African-Americans, women, and students of limited financial means. These students usually could not afford six to eight years of university education, and were often simply denied admission to medical schools affiliated with universities. At the same time, the Report tended to delegitimize existing women doctors and doctors of color. While many such doctors continued to practice, usually within underserviced clienteles, they did so under proscribed circumstances and for less pay."
And as I also mention there, From Marcia Angell: "The problems I've discussed are not limited to psychiatry, although they reach their most florid form there. Similar conflicts of interest and biases exist in virtually every field of medicine, particularly those that rely heavily on drugs or devices. It is simply no longer possible to believe much of the clinical research that is published, or to rely on the judgment of trusted physicians or authoritative medical guidelines. I take no pleasure in this conclusion, which I reached slowly and reluctantly over my two decades as an editor of The New England Journal of Medicine."
So, while the IAS is indeed a wonderful place as far as it goes, it is too bad Abraham Flexner did not advocate more broadly on the topic of the value of a diversity of knowledge and a diversity of exploration by a diversity of explorers -- especially in the medical field.