as another person who worked on personalization I can assure you that way over 90% of users prefer personalization. Decades of A/B tests have proven that.
And yes, some people, myself included turn it OFF because of privacy concerns. But we are in a really small minority.
I don't work at Youtube, but different teams probably optimize for different things. Time spent is really, really hard to change directly, so they probably focus on things like p(like), p(watch > X% of vid), p(watches another video).
Generally speaking there's no answer in principle to the question "is the time spent actually valuable". For the vast majority of people, time spent on Youtube cuts into time spent doing basically the same thing somewhere else. We're not all researching cures for AIDS or new sources of green energy. Most people just want to sit around being entertained.
"When we A/B tested users against different kinds of personalisation, we consistently found that they liked personalisation"
If your non-personalised experience is stunted in functionality because the core UX of the product is biased towards personalisation, it will be unsurprising if people "prefer" personalisation.
And yes, some people, myself included turn it OFF because of privacy concerns. But we are in a really small minority.