What point are you trying to make here? That amateur writers are amateurs? That AI is only "often" an improvement over an amateur?
> Which is frustrating, because I have the feeling the novel I've been trying to finish writing for the last decade may be usurped by AI before I get my final draft.
This statement shows such a warped attitude towards art and the creative process. What do you mean "usurped?" Do you actually believe that LLMs will overtake humans when it comes to creative works?
If so, you don't really understand what is compelling about the written word or what makes for good writing and reading and it's no wonder you feel as though your own writing is so substandard.
I highly doubt your writing is that bad. Especially if you've been working on it for a decade.