The force is a living thing. It's going to balance itself out over time. Once Vader killed the emperor and then died he left a void of power from the darkside. It might not have happened quickly, but the rise of force sensitive people, especially with one as powerful as Luke was supposed to be would cause the force to react in a way that would cause a being of darkness to rise.
The point of the trilogy wasn't to wipe out the force, but to bring back a balance that had tipped the scales into chaos. Now we have a new set of movies explaining how that balance will once again be restored by adding a darkside flavor back to the vanilla that is the light side.