I'm able to quickly make entire complex interaction flows happen smoothly and maintainably on a single page. Now, there's a point, where when you have to start maintaining the state of various widgets on the page with each other, it gets pretty iffy and its helpful to bust out React. But even then, it's actually usually quicker to prototype out the templates/api for the widgets using vanilla Rails.
I've found that, across many projects with many people, in the long run, these JS helpers are amazingly helpful and clarifying. So I'm going to have to firmly - but respectfully - disagree with your opinion that Rails shouldn't include them, simply because your opinion doesn't provide an alternative that gives me better benefits than what's currently there. It just strikes me as though it will divert engineering on actual hard problems of value to a lot more of simply engineering for engineering's sake. I'm not saying I won't change my mind...I'm saying, show me something better, and let's talk!
At a glance, I don't think you can really claim that the JS frontend world is mature or stable, so in the meantime muddling-along seems equally valid.