My idea is that they didn't/don't havse a choice but to try something.
They are probably running out of money and no one is giving it to them in this econimical climate.
Maybe profitability, or at least drastic measures, is the request by investors (similary how IMF is "blackmailing" countries when they give them loan).
It might also be an experiment, planned or accidental to male profit of social networks after 20 years of investing.
It is a gamble, but cut has to be made at some point.
If they really want to prevent us from watching videos without ads, they can. They know of the ad is watched or not, we can have some kind of auto-mute-during-ad but that's it.
Question is if they will kill network effect with it.
I have already drastically reducedy yt watching because of too many sponsors... watching two minutes of sponsored material, plus two ads just to see that I don't even wanna watch the stupid video is too much.
Not to mention those laud ads in the middle of relaxing and quiet video... few months ago one ad was starting with screaming, that's when I said no way.