Since all people mix together ideas they've learned from their own input to create new things, just like AI does, then all people-produced content should also be inherently uncopyrightable, unless produced by a person who has never been exposed to copyrighted content.

While copyright and IP law at present is massively broken, this is a very poor interpretation of the core argument at play.

Let me break it down:

• Yes, all human created art takes significant influence - purposefully, and accidently - from work which has come before it
• To have been influenced by that piece, legally, the human will have had to pay the copyright holder to; go to the cinema, buy the bluray, see the performance, go to the gallery, etc. Works out of copyright obviously don't apply here.
• To be trained in a discipline, the human likely pays for teaching by others, and those others have also paid copyright holders to view the media that influenced them aswell
• Even thought the vast majority of art is influenced by all other art, humans are capable of novel invention- ie things which have not come before - but GenAI fundamentally isn't.

Separately, but related, see the arguments the Pirate Parties used to make about personal piracy being OK, which were fundamentally down to an argument of scale:

• A teenager pirating some films to watch cos they are interested in cinema, and being inspired to go to film school is very limited in scope. Even if they pirate hundreds of films, it can't be argued that it's 100 lost sales because the person may have never bought them anyway.
• A GenAI company consuming literally all artistic output of humanity, with no payment to the artists what so ever, "learning" to create "new" art, without paying for teaching, and spitting out whatever is asked of it, is massive copyright infringement on the consumption side, and an existential threat to the arts on the generation side

That's the reason people are complaining, cos they aren't being paid today, and they won't be paid tomorrow.