Optimising YouTube viewing time is a terrible goal to devote your life to in my opinion. I'm not sure if OP ever thought too deeply about what he was working on, but addicting people to a screen is not one I consider a value to society.
I can’t wrap my mind around those goals, because (IMHO) the number one thing that would improve viewership is entirety outside of YouTube’s hand: content quality. So the author can only ever move needles that are thrice removed (i.e.: improving backend tools -> creators have more time -> creators create better content) or are statistical coincidences (we made the buttons round -> viewership went up 0.2%).
