No, sorry, that doesn't work: you're saying that by reducing incentives for primary work authorship, we'll gain more works because people will be able to work on derivative works based on existing ones. The only plausible way that could work is by allowing people to derive works from popular properties; literally: you're talking about fan fiction qua fan fiction. Star Wars. Twilight. Spider Man. Enabling people to derive new works from obscure primary works wouldn't move the needle, and wouldn't support your original argument.

I'm saying: I don't think you'll get enough commercial Spider Man fan fiction to offset the damage. But you can at least connect the dots on that argument.

Recent Star Wars and Spider Man stories were just fan fiction with permission.

Star Wars borrowed from many sources. Some inspiration and some copying. Most were obscure to most of the audience.

They didn't say most new works would be commercial. But it's plausible fewer very expensive works could benefit less expensive works.

You are free to borrow from those same works, have your superhero bit by a radioactive arthropod, or invent a coven of space witches with energy swords, and you'll be just fine.