Sanderson is missing a larger point: What do the audiences think?
If you give a whole lot of readers ten books, three of which were written using AI, and the readers can't tell which were AI and which were human... Do they even care?
I mean, we care because it's interesting. But if the readers couldn't tell the difference, then why does it matter? They were entertained, right? Assuming that the books were just fiction.
Does it matter because machines are now just as good as people? Or does it matter because now less people will go into writing as a career (the capitalism argument, not, "we just should have more writers")?
If a consumer is looking for good books to read that cover their ~~fetish~~ interests, and there's now 10x more books in that category than there were before AI... Is that bad? Forget quality for a moment and just assume they're all great because it's the future where all the kinks have been worked out.
When we don't need to rely on a handful of excellent, highly promoted writers like Sanderson to give us excellent stories... What have we lost? Anything?
Perhaps what gets lost is capitalism. Because it takes a lot of capital to produce and promote books.
New writers—even if they're fantastic—may never get the break needed to be picked up by a big publisher that has excellent, well-paid editors that can make a good story into a great one. However, such a new writer can use AI to fix their grammar and greatly improve their craft: their writing and the marketing necessary for people to find it and read it.