If stuff disappeared (even just the Youtuber rage quit, not necessarily end of the platform) and people were talking about being bothered about it I tend to think people like you would pop up with archives.

It's the really niche stuff that few if anyone would notice or care enough to talk about that would be properly lost. And if it's niche but there's a lot of care from the few, then that's one way that archivists are made.

You are way too optimisitc. Even for videos that thousands of people have saved, chances are you won't be able to find any of them.