The vast majority of paid writers are just summarizing other people’s thoughts. Like people writing marketing blog posts are just writing what the CEO tells them to, etc.
All of that just got way easier to do. A lot of it can now be done by people who couldn't have, before, because a machine can fix their writing so it's sufficiently non-shit to be acceptable for many purposes. The rest, that still needs some decent writer or editor involved, is likely to see a huge increase in per-worker productivity, which will either result in similar levels of employment with far higher total output, or... in most of the people doing it now, having to switch careers.
My expectation is that we'll just turn editors into writers-who-also-edit—aided throughout by AI tools—and have non-writers take on writing as part of their job (see: what happened to secretaries—we're all secretaries, now) and most of the writers are going to be out of a job. I reckon this'll hit commercial writing (where most of the writing jobs are) and high-volume-low-value fiction writing (e.g. the romance novel scene, middle-grades shared-penname book series, that kind of thing) hardest. Ghostwriters, too—one who embraces AI tools and uses them well will be able to do the work of five or more who don't, so we'll need far fewer people doing that work.