Again, though, he *already told* a condensed and reformatted version of that story in ROTJ. Having a new "twins of the Force" and a takedown of the evil Emperor does nothing but undermine (or at least regurgitate) the story that Lucas had already told.
I disagree but to each his own.
Alternate versions of what a creator would've done with his IP is literally what fan fiction is.
Not quite. First off SW is no longer George's IP. Second you'd have to apply that erroneously broad definition of fan fiction to hundreds of sequels then, even those that are literal classics like ALIENS. Cameron completely retconned the life cycle of the alien to allow for his queen, something Scott and the original writers never intended. Now you can dismiss the sequel from your own personal canon but it ain't fan fiction. Hell by your definition you could even write off portions of films like ESB itself as fan fiction due to improvs and ad libs that George initially didn't like ("I know," etc.)
Look I'm not an idiot, I know why you guys are calling the ST fan fiction. It's just slang for "I'm so outraged by these films not telling the story I wanted to tell that I'm going to give them a label that objectively invalidates them!" But it ain't gonna work, sorry. If you don't like 'em then you just have to ignore them like so many have done with most of the EU and the PT.