I get what you’re saying, but YouTube music is pretty much just a different front end for the normal site.
Sure, it does some filtering to attempt to be music only (though I’ve seen non music stuff sneak in before) but in the end, you get pretty much the same core experience if you open up the YouTube app and start “watching” a song (with premium for the background play capability).
I’d be willing to bet this is why they won’t go the route you’re talking about.
I’d prefer some kind of limited amount of viewing. I don’t watch a ton of YT, so give me some kind of reasonable ad-free cap. I’m willing to pay to not see ads, but I don’t watch enough to be worth their asking price.