And Grievous wasn't a sith.
He was an army general, who was changed cruel by the cocky Sidious. He knew how to use a light saber, but he didn't understand it l ike a Jedi or a Sith do. He used it as a weapon, we use it as an extension of our bodies. The first light sabers were used in ceremonys, not for fighting.
The Jedi were ignorant and kicked the Sith out. Instead of dealing with them properly, even if it meant a battle or two.
Peace. Much good that did for them. Cost the lives of entire planets.
The Jedi are blind with invisible power.
We Sith are blind with very real power
And if there wasn't Jedi, there wouldn't be a dark side. Do we really need any of them?
Oh, and the Sith were never extinct. Back in the early republic they simply kicked the Dark Jedi out of the Republic. And after the death of Vader there was a Sith army.
The Sith have never been 'extinct', just hiding.