When I think, "AI slop" I think of things like fully automated tweets, LinkedIn posts, blog posts, etc that are literally created from prompts like, make sure to post about topic X at least once a day. OMG, that shit is toxic waste as far as I'm concerned.
...but cover images? Who TF cares‽ If the story is successful, they'll replace that image with something superior made by a real artist. In the mean time, they were never going to pay an artist anyway so it's not like it hurt artists.
Having said that, there's one very specific industry that has already been completely destroyed by AI: Stock images. Getty? Nobody pays for that anymore when they can just generate something with AI. But how upset can we be when stock image companies are such highly exploitative corporate entities?
For me, it's like someone vibe coding an alternative to Photoshop with minimal effort. Like, yeah, I'm sure it's missing loads of features and has tons of bugs but it hurts Adobe (a proper villain) so let's go! Haha.