This whole discussion just serves to remind me of Lucas' ineptitude at crafting a cohesive "cosmic" story that makes any sense.
George's party line is that when the two Sith personified in the beings of the Emperor and Vader were killed/redeemed, evil itself was pretty much destroyed, a horrible imbalance created by the Sith's use of the Force was corrected, and a possibly-eternal period of peace and prosperity was ushered into existence throughout the universe. So, how the hell does this exactly work? The Sith can't misuse the Force, since the dark side is an inherent aspect of the Force that exists independently of any user who may choose to weild it. Even if two darksiders could profane the Force, so to speak, how come the thousands of Jedi who used to exist - who supposedly used the Force in the right way - couldn't use their powers to offset this disturbance somehow? And anyway, assuming all this claptrap could be made to make sense, how does the eradication of two Sith prevent other non-Sith darksiders - either from within or without the galaxy - from coming into existence? Logically, unless the Star Wars universe doesn't extend far beyond the main galaxy, there will always be darksiders elsewhere in the universe to misuse and unbalance the Force.