I’m not sure I understand that reasoning besides the “upset at not getting what you want.” TFA introduced a lot of hanging threads that could go one way or another (JJ has admitted to wanting to keep things as open as possible for the next person). Rian was very free and open to do a lot, because of how open it was and because there would be a film after his. It would be unreasonable to suspect for the second film to cover everything, nor would that be interesting if all stones had been overturned. So it was 100% his prerogative to say “it doesn’t look like the Knights of Ren fit into this story in a satisfying way” (the irony of course being that his version of “unsatisfying” would have used them in a way that would ultimately have been more satisfying than what we got, sadly). But TROS is the conclusion, and so necessarily has to follow the dotted line of what’s been set up in the last two films without punting any elements for later as those two films had the liberty to do.

Using the Knights of Ren as the big bad instead of Palpatine seems so obvious it’s truly baffling it’s not what we got. The idea that they would have been too much to set up in one film rings false when TROS as is takes devotes nearly all of its runtime to the reestablishment of Palpatine. Not to mention the big bad in ROTJ doesn’t have a lot of screen time prior to that film either (not a perfect comparison but you get what I mean).

Ultimately intellectually I still don’t have a problem with a different creative mind taking on each film in a trilogy, but in the end it definitely didn’t work. Part of we thinks now there maybe should be some sort of continuity between part 2 and 3. If you don’t fully understand what the second act sets up, you can’t properly finish it off.