You guys keep ignoring the part about giving these guys plot immunity is the problem that I have repeatedly said. Making Darth Vader immune to anything the players do to defeat him regardless of how effective it would actually be. Player hits Vader with a bar a dim bomb and succeeds in hitting and the GM say nope he survived...that is not cool. That is crappy gming.But if you are protecting elements from the players you are kind of being a crappy GM. Your players should be able to run over canon. If it makes sense. If you build a solid version of Darth Vaders and the players meet and kill them. You should let Vader die. And whatever happened in the movies is now changed.
Do what you want for your own games, of course, but don't call out GMs for being "crappy" if they want to make their Star Wars universe the Star Wars universe. The Star Wars universe is defined by more than its tech, and for some of us, saying "oh, Luke is no longer important, you players are the super-best" takes away from Star Wars.
And, on a broader note - in any story, your characters don't have to be the center of the universe for it to be a good story. Some players want to be that in their stories - fine, that's okay. Other players (and GMs) find the notion of making characters "central" or "the most important" anathema to the idea of a big universe.
If plot immunity is your only point, then I agree. But that's not what I, and apparently others, are reading into your point, because that only represents the tiny finale of what should be a pretty big story arc. There seems to be more behind your point about immunity.
Killing Palpatine requires the GM to present Palpatine. And I have no intention of presenting Palpatine, or really any other canonic figure, because that's not what the stories (so far) are about. Our stories have nothing to do with any of the canonic figures. We play little enough as it is, I have no desire to enter into the uber-geek-zone. So if that's what makes me a crappy GM, then you're trolling.
Now if my players were Star Wars savvy, and they all said "we've got 1000XP now, we want to go kill the Emperor", and they could present a good story reason and were committed to doing the story justice, well then I'd have a lot of work to do, but if they pulled it off, so be it.