Option 2. is unstable - if you split users into two camps, "watching ads" and "paying", it so happens that it's the second camp that the advertisers want the most to reach: people with disposable income and willingness to spend it. You can expect significant pressure from the business side to somehow advertise to that second group.

This is, in fact, a point raised by many commenters in the thread - it's very likely that Premium will stop being ad-free once it gains a large enough user base. See also: cable TV, or any other paid-for medium whose initial value proposition was... "we charge you for it, but there are no ads!".

Option 3. - nah, can't imagine it either.

Option 4. - that would be going back to the "good years". I don't think it's possible for YouTube - they'd have to somehow undo or reverse the pressures that got them to the situation today. See also: image hosting service cycle - they start bullshit-free, then they get progressively worse, but it's only when they go past the "nerds block ads, whales pay, suckers get ads" stage - when they disturb that balance - that they die.