Anakin fulfills the prophecy by slaughtering the Jedi. When ANH rolls around it's only Obi-Wan and Yoda, balanced by Vader and Palpatine. Luke's mission is to bring Anakin back to the force, this is the beginning of the end for the darkside as Luke becomes the most powerful Jedi ever and he will ***** slap and dark jedi or sith in his way.
The balance of the force refers to those who still know of the force and communicate with it. Vader and Emperor are the two who handle the dark while Obi-Wan and Yoda are the light.
Why do you think so many Jedi die?
Besides, even if sith aren't around, the darkside is still present and possibly more powerful so Anakin brings balance by destroying the jedi order.