I'm not 100% sure I agree. They're celebrating, and really happy, but they celebrated and were happy at the end of 'Star Wars' also.
The celebration at the end of Star Wars was of a different kind with a different tone. The ROTJ celebration was an end-of-the-war it's-all-over we-don't-have-to-worry-about-the-future kind of celebration.
Unless we're counting the novelizations as the same as the movie, which I don't. They're EU too, to me.
The novelization saying the empire was dead demonstrates what was the official interpretation of the film's ending back then. And it can't be coincidential that the Marvel comics and everybody I knew and Lucas in the SE all agree with the novelization. It's irrelevant whether the novelization is EU or not. What matters is that it demonstrates the official view. The novelizations have always been the foremost companions to the films and adaptions of the films and Lucasfilm canon has considered them higher than all ordinary EU since at least as far back as 1994 (1994 was when Lucasfilm first said anything about canon). I don't buy everything in the novelizations, but I do recognize that they are taken more seriously than ordinary EU and that they were made with a pretty serious intent and often include stuff not in the films that comes from Lucas and include stuff that shows the official interpretation of the films. If the novelization back in 83 said that the empire is dead at the end of ROTJ that means the people at Lucasfilm figured the empire is dead at the end of ROTJ. The novel wouldn't go off on its own over something so big as the empire being over. It would have to be pretty recognized officially for them to put that in book. And what do you know, Lucas demonstrated that it was his view too (in the SE) and the comics too. The only thing that disagrees is the 90s EU, which was a revisionist take that came along long after the film came out. I think with all that evidence you have to be really pushing it to make out that the empire wasn't meant to be over in ROTJ.
I think the novelization is telling us the original intent behind ROTJ's ending.
Endor was a major victory, a pivotal victory, but they never say in the movie it HAS to be the final victory. Mon Mothma doesn't say 'If we win this, it's over.' Alternatly, I see no reason to insist the Empire, or some form of it, would continue, but I also don't see any die hard reason in the movie to maintain that all the fighting is over.
Emotionally the movie DOES give you the message that the empire is over at the end of the film. Yes the movie does give the impression that the fighting is over -that's the tone of the celebration and ending scenes. And Mon Mothma may not have known that the Emperor and Darth Vader were both going to be killed in the battle of Endor. After all, Jerjerrod was pretty surprised when he heard the Emperor was going to be coming to the Death Star.