This does not align with what I see in the fiction book space at all. At least in the communities that I frequent (fantasy, sci-fi, horror) there is no AI publication that I am aware of that is good. And readers react allergically to any notion of AI involvement be it for covers or the actual writing. I think people will gladly pay a premium for human written texts just like people gladly play chess in fact the number of chess enthusiasts has drastically increased despite engines being able to crush the best players in the world. Writing is art and art requires a human soul for lack of a better word.
As for proving that you wrote it...yeah that could be tricky. I do all my writing in Latex tracked in git. I guess if the AI accusations get really wild one could add some sort of proctoring for the writing process (film yourself while writing).