Everything Luke, Leia, and Han fought for has been forgotten, undone, and now we are supposed to expect the new characters will somehow fix it permanently this time.
Nothing is ever permanent, especially across an entire galaxy. Just because there is a new conflict doesn’t mean the fighting for a previous victory is for nothing.
That might make sense from a realism point of view, but it doesn’t work from a film-making perspective. The Original Trilogy has become irrelevant.
Will the conclusion to this trilogy be a similar defeat over the Empire? If so, then it stands to reason a new force will rise up in 30 years and overthrow that. The Saga has to end with Episode 9, but if they don’t give us a proper reason to believe this is it, it’ll fall flat on its face.
A “unification” between dark and light sounds equally disturbing. It isn’t like there are two extreme factions each bad in their own way. One is good and the other is bad. We see the First Order commit mass genocide on multiple occasions. We see the Resistance save the day on multiple occasions. Even if Kylo decides to merge with the Resistance, it will appear as if the First Order won, because the Resistance is already perfect and all they can do now is corrupt it.
Now that I’m thinkin about this, I don’t know if a solid conclusion to Episode 9 is even possible with the hole RJ dug us into.