When rightsholders give notice of alleged infringement, YouTube can't simply be like "well, they say it's fair use, so we're going to ignore this." If they did, they'd be knowingly contributing to copyright infringement and lose out on safe harbor provisions.
I didn't propose that, so I'm not sure what point you're making.
I just said that YT should receive and acknowledge claims of fair use or public domain status. Then, if a copyright claim arises, video uploaders know that YT is already aware of the asserted status and will evaluate it alongside the copyright claim, instead of assuming the latter to be true by default.
