In a way you are correct, Kingdom Hearts II was like Return of the Jedi: the story could have ended here, every major plot point had been resolved, the protags are all reunited, the bad guys are all defeated, roll credits. But then Nomura added the bane of any franchise: a fucking prequel, which added a shitload of plot points that wouldn't be resolved for nearly a decade (hence why KH3 was so anticipated). And then Dream Drop Distance came out, added fucking time-travel to the plot and negated every single achievement the protags had ever accomplished by reviving all the main villains with little to no explanation because "it was all part of my Master Plan my dudes".
3D is what turned the plot into a fucking joke. They could have had their final battle and resolved the plot points left hanging after the prequel without that fucking time-travel shittery.
And with those rumors that the next Star Wars will feature time-travel, and both franchises being owned by Disney, clearly they just never learn.