Yeah, that's exactly it. I think remixes might theoretically work if it were fully negotiated as that, with full co-author credits and an author's note generally going "[name]'s story was so awesome that I desperately wanted to rewrite it my way and present it all to you, and [name] was kind enough to agree." and pay commensurate with being the co-author. But it would always be terribly prone to exploitation, just because the gap in power between an amateur fanfic author playing in someone else's universe and a published and well-known author making their living off that universe is, well, very large.