Originally posted by Rogue Jedi
If Sidious had killed Luke, he would have kept Vader as his second for the time being, but they would have perished when Lando and Wedge blew up the death star. either way, the Sith would have been exterminated.
IF I may cut through this movie-only thread with a bit of post-movie insight...
If Luke and Sidious had died, the Galaxy still would have been the Empire. In the Dark Empire comic series Sidious returns, the Luke-founded New Jedi Order and Luke himself do quite a bit to stop Palps, and without them around the Empire would rule the Galaxy once more. Of course, then the Yuuzhan Vong would come, but the Empire would be able to defeat them with less effort than the New Republic. The Empire would once more be the rulers of the Galaxy.