But they could potentially make a larger steady income stream if they also had ads. It happened to cable TV, it happened when HTC introduced ads to their tiles or timeline or whatever it was called, ordering food online you'll get spammed when you try to checkout to buy their new promotional item (even if you already have it in your cart), and so on.
If ads aren't a perfect fit then they'll still try to add other revenue models to things you've paid for -- smart TVs selling data about what you watch or which products your conversations are about, car diagnostic products shipping a patch to only work if you start shipping them location data, large companies like Garmin buying up smaller companies and cancelling any lifetime accounts in favor of a subscription model (and vehemently denying that it's a money-grab), needing a Facebook account to use your Oculus, and so on.
If they're able to extract more value from you without excessive negative repercussions then they will, independently of what's "right" or legal.
To try to answer you anyway, they've put a fair bit of engineering effort into making YT mildly painful to use without Premium -- things like not allowing sound to play when the screen is off (and covering your screen in ads for Premium if you wake up the phone to a running YT instance). I'm sure some people pay for Premium to improve that experience.
You mean for users not paying for Premium? They will almost certainly not do that, since fewer watches means fewer ad impressions, which means less revenue for them. Plus, they don't want to push away the users that spend all of their free time watching YouTube videos.
