There's an important distinction between possible outcomes here though.
Sure, if Luke had failed to defeat Vader, or had been killed or had struck Vader down and fallen to the dark side it probably wouldn't have made a difference (speaking in terms of the film canon alone, lets ignore the EU with Emperor clones and Luke's future) because the Rebellion would have blown up the Death Star and killed everyone on board.
However, if Luke hadn't been involved in the story at all - if he had stayed on Tatooine for example as the original poster suggested - then things would definitely have been different, in all sorts of potential ways.
If Luke hadn't been on the Death Star fighting Vader and Palpatine, then the Emperor may have focused more on personally overseeing the springing of his trap to crush the Rebels. He might have personally countered the Endor assault, or had Vader do so. Or at the very least, he would have been able to evacuate before the Death Star was destroyed. Hell, he might not have even been on the Death Star at all if not for his intention to confront Luke alongside Vader.
And that's not even thinking about what might have been different in the previous films. If Luke hadn't been around, would Obi-Wan have sacrificed himself against Vader? They probably wouldn't have rescued Leia, she'd have been left on the first Death Star and presumably killed when it was destroyed. That's if the Rebels were even able to destroy it without Luke... which they probably wouldn't have.
Overall, Luke played a pretty important role in the film even if you don't take into account the "true" story of the film, which was really about Vader's redemption. Sure, looking at the overall fate of the galaxy he didn't directly destroy the Empire himself. But things would have been very different without him.