How would they pay for the infrastructure required to support all those users? I can't stand ads, but when I was younger, no way would I have paid for YT Premium (though to be fair, ads are much, much worse now).
Subscription services are like hungry hungry hippos, you give them $10 a month and next year they want $100.
I honestly think if everyone starts paying, it will only make them remove the free tier quicker. I think society is better with youtube free, even if ads are annoying.
Enabling on site money transfers (as YouTube does) and taking a small cut from each transfer (far less than YouTube's lol level 30% cut) would probably be getting close to enough to cover your costs, especially if you made it a more ingrained/gamey aspect of the system - e.g. give big tippers some sort of swag in comments or whatever, stuff like that. It's not going to be enough to buy too many [more] islands for Sergey and Larry, but such is the price we must all pay.
[1] - https://www.pcgamer.com/gaming-industry/valves-reported-prof...