Most fanfic writers do have their own story they want to tell. They may be borrowing characters and/or setting, but they do need their own story, otherwise the writing would be boring.

I wonder where these "original" writers would place those who write professionally in established stuff, are they not real writers either? I'm thinking for example that modern day Sherlock TV adaption, or even the new Star Wars movies (let's face it, most people who write those were probably fans of the original). Or the writers who write Doctor Who (who were fans of the original series), or people professinally writing established comic book characters? But I guess they would probably look down on all of those too, even though that seems a bit ridiculous to me? Those people are clearly writers?

Obviously self published fanfiction has a much wider spectrum of quality than professional stuff, but that's the same with self published books too.