Personally I think it is a huge error we don't force archive.org to allow others to mirror their data easily.
What do you mean by this? I thought they definitely were open to mirrors, why would ’we’ need to ‘force’ them to anything?
IIRC the Bibliotheca Alexandrina in Egypt had a mirror of the web archive up, though they may have failed to maintain it
I can already look at e.g. the National Film Registry and say I don't care about movies so my tax dollars shouldn't go towards preserving them, regardless of what some bureaucrat thinks is "culturally significant." The film industry has a pretty massive network of artists who are able to go to bat for that stuff, though.
But then take pornography, or hate speech (and/or straight-up misinformation), or content that's illegal in the US but not elsewhere (like drug recipes or weapons schematics). It's already tricky for a private entity to handle convincing people of the value of preserving everything physically and legally possible to preserve. Making an argument for doing it as a public function, even in good faith, could easily turn into a mess-- especially when we can barely agree what should be allowed on the internet now.