George is a pretty bad writer, and unfortunately by the time of the prequels everyone who worked with him was scared of him. So no one told him anything about the scripts didn't make sense, let alone take it out of his hand and force rewrites using professional script writers. He has however stated that the idea of bringing balance to the force was destroying the Sith. Basically he meant balance as a vague term for "good outcome." Obviously there are some holes that can be poked in this, in that it has nothing to do with "balance," and it seems pretty unlikely that evil Force users were put to an end just because Darth Vader threw Palpatine down a shaft.