Archive.org have stated they do not want to be a complete mirror of YouTube which is fair enough. Content creators could easily retain a local copy of files and upload them to archive.org in the even of removal, or an individual could retain local mirrors of channels to do the same thing. Archive.org are aware of the fact that things hosted on websites might disappear, I can’t speak for why YouTube is not a desirable thing to mirror but I assume the volume would be massive and the risks of potential copywrite / illegal / highly illegal content etc would be undesirable from a management perspective.