Sith rule through power and leaders only change when they become overpowered. This creates a constant need for strength and competition.
However, the empire itself is merely a group of controlled areas ruled by fear. If the Sith Empire stood still and did not attempt to keep coming back and starting war after war with the republic then it would die out. Fear from people in your own lands will only carry you so far. It keeps people in check for the most part but resources and productivity would eventually dwindle to the point where the empire can't survive. This is why they are constantly fighting to expand control to new worlds.
However, because of their expansion, they gain control over more and more people. The more people they control the more people they oppress. Eventually they pressure people so much that they explode into a rebellion that quells their expansion and forces them into a retreat.
And so is the universe of Star Wars.