I would argue that popularity is actually a great indicator of the quality of an OS and your experience with it.
I am what you would consider an "expert" (or at least, I can definitely competently administer a linux computer) but I don't use linux distributions for day-to-day computing. The reason is that linux computers, even ones running "popular distros" such as Ubuntu, are considerably more prone to interoperability foibles and installation/updating problems.
The reason popularity matters, even though I have the technical skill to administer any OS competently, is that popularity is the single best incentive to encourage third parties to iron out the kinks on a particular platform. The reason AMD drivers work better on Windows than they do on Linux is not because Windows is "better" by any of the metrics of quality you espouse, it's because Windows is more popular. The reason Steam for Linux is only supported on Ubuntu is not because Ubuntu is "better" it is because it's the most popular distro. The reason to use popular platforms is because all of the little things that require a half-hour of expert time to work around would have already been solved by the vendor who recognized that such issues are worth testing for and fixing for a platform with 100 million users, but not for one with 20,000. That's distasteful to some because it doesn't seem meritocratic and it's out of the control of the creators of the OS, but it's a fact of business, and it does affect the quality of an OS in tangible ways.
As for the "expert" thing, the costs really aren't that different for various kinds of users. The reason I care about this stuff is because the opportunity cost of my time spent solving menial issues that would have been solved by the vendor if I were on a more popular platform is too high. The reason my mother cares about this stuff is because the transaction costs of finding someone who will spend the time to figure out what's going on and fix her computer is too high.
> The reason popularity matters, even though I have the technical skill to administer any OS competently, is that popularity is the single best incentive to encourage third parties to iron out the kinks on a particular platform.
I prefer a working platform that forces me to read the manual beforehand over a platform where I don’t have to read the manual but random third parties iron out kinks.
Popularity is a measure of quality, but only of quality wrt to the usecases of the people among which the product is popular. My computer usage is very different from my mum’s, and likely from 95% of the population, hence, the popularity of a product among 95% of the population is rather irrelevant to me.
If you’re a single person with a tight budget looking for a car, you don’t look at car popularity among large families owning oil fields, but among people similar (in the relevant ways) to yourself.
Hence, popularity among a large user base is an indicator that the OS is optimised for use by a large part of the population. If you find that this is indicative of the quality you’re looking for, fine.