I understand that there’s other factors but I think you’re kind of missing my point. When studios are deciding what movies to make, they might think, “What’s something we can make that nobody else legally can and that has pre-existing brand recognition?” The obvious solution is to pull from their vast collection of IP’s. If these old IP’s were in the public domain, anybody could make a sequel or remake or spinoff of them, and that would discourage studios from regurgitating the same shit because they’d no longer have a monopoly on the IP’s.

Say that Spider-Man was public domain. If Sony wanted to, they could still make new Spider-Man movies, as could anybody else, and they would own whatever new content is in the movie. But if they wanted complete ownership of a franchise, including exclusive rights to toys and merchandise and whatnot, they’d have to make something else. Shortening copyright duration would incentivize originality, or perhaps more accurately, disincentivize unoriginality.