> Much of YouTube content is (perfectly legal) remixes, responses, or criticisms of other YouTube content that embeds part of the referenced video in their own video.
As weird as it sounds, you are wrong. In a short summary:
1. Fair use is a very small exception to some very broad rights.
2. Fair use almost definitely does not apply to most youtube content : if you use content other people made (video games or music immediately come to mind) you are infringing copyright holder rights.
3. If you rely on fair use rights, then you might find yourself in trouble.
What's even worse is that right holders can pick and choose who to bust, and they don't need to be consistent about it. So even if they rarely go after small groups, they can still shut down bigger ones.
I have a childhood friend who is now a copyright lawyer and I sometimes jokingly ask him whether something is copyright infringing. Other than yes, the most frequent answer I get is "I don't know, it depends. Both sides have arguments so ultimately it's down to the judge." It's just bad law. The only difference between then and now is that now we have the technology to actually enforce it.