Subscriptions are key to making a predictable revenue, which shareholders love. They are also key to making users even more captive than they already were. If you pay a subscription, all the more if it's paid yearly / has a yearly commitment, you're more likely to stay on the platform and use it, rather than go see elsewhere.
Whether it's going to "work" for YT, I don't know. We'll see. You'd think it would backfire and drive users away, but "unfortunately", I'm not too sure about that. People are already hooked, and alternatives are few and not quite up to par. A minority will leave, a majority will pay. Probably.
OTOH, if say people buy subscriptions, that allow bypassing all ads, en masse, then it will drastically decrease the ad revenue. And drive advertisers away. Fun shit. So the question is, how can they screw up both sides as much as possible in order to maximize profits? That'll take a few MBAs to figure that out.