In Return of the Jedi, Palpatine tries to turn Luke to the Dark Side. He fails, and decides to kill him, but instead Vader intervenes and the rest is history.
But, while all of this is happening, the Rebels manage to destroy the shield generator on Endor (with unexpected help from the Ewoks), therefore allowing Rebel space forces to destroy Death Star 2 itself.
My question is, what would happen if Vader didn't kill Palpatine (for one reason or another)? Events in the throne room seemingly didn't affect battle that was going on. EU Legends mention that Palpatine used Battle meditation, but as far as I know there is no confirmation for this in canon.
Without that, I don't see any reason why the Rebels would not destroy Death Star 2 anyway, even if Luke failed in his personal quest to confront Emperor.