In my mind Palpatine should not have returned and I doubt that’s something Rian would have envisioned as the conclusion to the story he set up in TLJ. I should be clear, just because I wouldn’t have done it doesn’t make it a bad decision. But again, when you’re at the conclusion of a story, you have to necessarily follow what’s been set up. I think JJ would argue Palpatine was there because it was a conclusion to the whole saga, and that’s totally fair. Maybe there was a way to do it that would have worked. But I don’t think you can look at what they did and say it seems like a natural follow up to the previous two episodes, or the previous eight.
Personally it seemed to me like Rian’s eye was very much on the ball for what the trilogy needed at large, in particular, he saw the trajectory the Snoke/Kylo storyline was on and decided to expedite that so that the trilogy would be forced into a position in the third film where it wasn’t just aping ROTJ (whoops). I think you can draw a line from that decision to not finding room for the Knights in TLJ. Rian made a choice to have Kylo forge a different path than what we got with Vader. Whether it’s what you or I or JJ would have done or not (and to be clear, it’s probably not what I would have done - that’s not the reason I like it), in the second film he very much had the freedom to make such a choice. The failure then ultimately I feel lies with JJ for refusing to “play ball” with that decision.
Ultimately I should clarify too that I don’t think JJ was under obligation to use certain “elements.” Like, honestly I probably wouldn’t have even cared that much if the Knights weren’t in the film? Just would have been a missed opportunity (so, no different than what we got). I think being bothered by that sort of thing is kind of the wrong way of looking at it. People complain about Phasma and say she was a waste, but I say who cares? She served a purpose in TLJ. Ultimately the problem is that the Knights in TROS didn’t serve a purpose. And JJ’s real failure is not that he didn’t follow through with enough screen time for them or Maz or whoever, but that he didn’t follow through in general with the character arcs and thematic trajectories from the end of the last movie.