Didn't Cadeus come back and restart the sith? (That's what I believe, I tried to read his wookiepedia page once, and it was just too long for me)
Edit: my point, the sith never lost in the end. "An individual may die, but our order is eternal"-Darth Zannah
Caedus was Sith, yes. But he never really restarted the Sith. He didn't seem to last that long either. Not with Luke Skywalker around
Edit: The Sith will always be around, but after Sidious' death the Sith have just never been that strong. Sidious was the pinnacle of the Sith so it can only go downhill.