The OT had a downward trend too:
ANH: $503M
ESB: $400M
ROTJ: $374M
It's just the nature of these things. Ep. II fell significantly form Ep. I because the excitement waned. Ep. III actually did better, but not more than Ep. I did.
Let me ask, suppose they had a"better plan" - this was ALL written out well ahead of time with a with a very tight narrative - but it was all written by Rian Johnson? Basically, three Last Jedi's - would that have been acceptible? It was all planned out, but Luke's story is the same. The line seems to be that it was bad because they didn't plan it - really it was just "bad" because they didn't like it - which is fine of course, but there's this idea that the quality is objectively bad and if they had done X or Y it would have to have been good. I just don't get that.