I agree--logically, the ROTJ ending makes no sense; yet emotionally, it was always quite obvious to me that the message conveyed was that the Empire was defeated, and good guys won. I mean you practically could have had

"And they lived happily ever after"

when the iris closes on the final shot. Thats the point--thats the message you get. They can't live happily ever after if ROTJ just amounts to a strategic victory, the message throughout the entire movie, emotionally, is that "this is the final battle--it gets decided tonight", which is why all the sacrifice and basically putting your eggs in one basket approach (ie send the entire Alliance in a last-ditch battle to destroy the death star).

Personally, i never considered that there was the Empire out there, and I never knew anyone that did either--the film says "the good guys won, the Empire is defeated." Certainly that is what Lucas was trying to convey, and I think it largely worked, even if it doesn't work in a real-life setting, but then Star Wars has always been full of logical holes like this. While we are contemplating why the Rebels are celebrating what is only a strategic victory, we might also be contemplating how they can be celebrating on a planet that should be having nuclear winter.