I'd bet the ratio of time I have spent legit learning something useful vs just using it as distraction/entertainment ("educational" channels are often just entertainment for nerds like us)/background, it has to be something like 1000 to 1. I wouldn't need to replace the 999 at all. I guess I would read books a bit more, probably get a lot more done on personal projects, go out a bit more etc.
Not clear at all my life would be worse off except in that pinch where I need to know how to disassemble & fix the thing, right now.
It is though. Videos with "limited ads" (as it's technically called in YouTube Studio) applied to them still get paid out of Premium views.
But for the most part - probably nothing. For everything else, it'd just be either some other free option, or like going back to the internet of the early 2000s, which would be good and bad in its own ways.
As for content, it depends what you're looking for. For me, I'm mostly into maths and physics and there are so many channels and lecture series that were immensely helpful. For example, I recently went through this playlist on Lie Groups [0].
[0] https://youtube.com/playlist?list=PLN_4R2IuNuuRgJb00X2J53Iq9...
That being said, lately YT has way too many ads for my liking; thus I am using Reddit more and more for these things.