Well, ultimately we know the Sith Empire does fail. Now when that occurs is open ended, but obviously sometime before the rise of the New Sith Empire c. 2000 BBY (founded by Darth Ruin, which will also eventually be destroyed circa 1000 BBY, followed by the rise of Darth Bane and the rule of two).
As far as I know there's a pretty big timeline/lore gap between Vitiate's Sith Empire (the one for the game) and the one founded by Darth Ruin. I suspect at some point in the timeline the emperor dies or is killed, and then you have a bunch of warring Sith "fiefdoms" until the rise of Darth Ruin, as the emperor's strength appears to be the only thing keeping the empire "united" (or more accurately keeping the Sith from warring with each other).
If we go by what Pre Vizsla said in The Clone Wars, the "Old Republic" and the Galactic Republic of the New Sith Wars and Clone Wars may not be the same entity either.