I've read hundreds of books, fiction and otherwise. This isn't a brag, it's just to say, believe me, I know what professional writing looks like and I know where LLMs currently stand because I've used them a lot. I know the quality you can squeeze out if you're willing to let go of any presumptions.
You'll notice that not once did I say current LLMs could wholesale replace professional writers anymore than they can currently replace professional software devs. I just disagree on the "not a good writer" bit.
If it's the opinion of professional writers you're looking for then you can find some who disagree too.
Rie Kudan won an award on a novel she used GPT to verbatim ghostwrite (no edits essentially) 5% of. Her words, not mine. Who knows how much more of the novel is edited GPT.