While I would agree with what you said. Originals do deserve their own categories but to say Fanfics are just run of the mill unoriginal work would really undermine what people write about.
I’m not supporting those who literally copy paste an original anime scene word to word with the exception of throwing their OC in the mix of it all. To further make it annoying some of them are reincarnated and they act all happy and chummy to fit in that specific worlds. That honestly makes me sick. If your OC is just gonna follow what the main character of the original anime would be what's the point. So yeah I support what you say.
But there are few very good Fanfics that are mostly AU. Sure they start of with same world but you could see a massive difference in characters their personalities and how the writer portrays it. Calling them unoriginal is a bit too much.
Just cause you have the same world does not mean you cannot make something original out of it. There are a million different ways things could in the same world affecting everything.
Just depends on what the writer is looking for. For me those who make everything look fine and Dandy about anime worlds and rinse and repeat the same plot over and over is where I draw the line