I think you touch on exactly why it's a bit disposable: they aren't breaking any new ground. But there's more...
As bad as the prequels are, you could at least get into the new alien species, new locations, new Sith Lords, etc. Trying to understand everything's place in the greater lore was interesting because it was all being helmed by the original auteur. Looks at the new pod vehicles! Look at the new robots! What the hell kind of alien is that? Look at the new force abilities we're being shown!
There's really no new ground in TFW; as you pointed out, retreads are abound.
People don't realize how difficult it is to take a shot in the dark and just make people believe this new alien race exists because you say so. That takes creativity, vision, conviction, and imagination. I don't really think anyone at the head of the franchise is trying to keep the lore growing in any kind of interesting way -- certainly not Rick Berman or Kathleen Kennedy. It's nobody's job to care about Star Wars at that level anymore.
It's the trade we got when Lucas bowed out; we got a Star Wars themed modern movie with all the trappings that entails -- political discussions about "mary sues" and cast diversity, depressingly forced cameos, dumb plots that make no sense, and shit-tons of CG. We also got a more earthy and realistic feel, because they figured fan-service was marketable, and we got better pacing because there was accountability were there used to be an all-powerful turbo nerd at the helm. But most of all, we got an utterly forgettable and shallow experience, which is Hollywood's speciality, and always has been, save for the work of the type of auteur that, for all his faults, Lucas was.
Edit: This all said, TFW is better as a standalone movie than the prequels. Can't get with you there. I never want to watch the prequels again unless it's bad movie night and I'm with some stoned friends.
