Simple for me, it renders the OT pointless in favor of just rehashing the same conflict all over again, only less interestingly this time. And it doesn't do justice to either the old hero's or the new ones either.
The Old Republic stood for a thousand years and it took a decades-long plan by a master schemer taking advantage of inherent systematic weakness while also orchestrating various crises in a slow but unstoppable rise to power.
Then 20 years of Imperial rule, 4-5 years of conflict in the OT ending with the toppling of the Empire.
Then there's a mere 30 years of peace (none of which we actually got to SEE BTW), during which we're told that the NR was corrupt ineffectual and got nothing of substance done (again the PT did this, and did it better, stop rehashing things). And then it fell, because some jerks led by an out of nowhere Emperor wannabe who we know nothing about, blew up a few planets and then every collapsed within like a day or two. Leia's life's work crumbles in record time, and her Resistance is all but obliterated within days as well.
Luke's JO (which again we never got actually SEE) is wipped out OFFSCREEN because apparently he learned nothing from the mistakes of his mentors and just did them all over again, and we have another evil Skywalker who kills them and goes bad and puts on a mask (again stop repeating things), and Luke just quits and ignores the rising threat to the Galaxy so that he can go pout on some island. And then he dies because, he got tired I guess.
Han loses his son, runs away from his wife (apparently because he doesn't know her at all even after decades of marriage), then his last act is to FAIL to get through to his son, and then he dies.
Again how is any of this "significant." How does THIS make the ending of ROTJ feel satisfying now? Basically the peace didn't even last the old character's lifetimes, they failed at everything in the end, and then they died pathetically while we're right back where we started 30 years ago.
Why are we supposed to find this interesting or satisfying?