The lore of the Sith does not work in any real world, even a game one. It's silly and cartoonish and doesn't stand up to logic.
Any organization that thrives on promotion by murder will consume itself rapidly. Darth Bob kills Darth Jane, now Darth Bob is weak, so Darth Suzy kills him, and so on and so on... It can only exist in a short-form medium such as a movie or tv show. The alternate-reality Evil Star Trek universe has the same issue. It just does not work on any logical level in a continuing medium like RP in a persitent game world. To play it in an ongoing world, it has to be played differently from the lore or it will fall apart.
I wish you would be quiet about it being cartoonish. It's really quite annoying and unfathomably incorrect. ALL of the Sith Empires have lasted decades at least and centuries at most. It works like any other civilization. For some reason, little known fact after the 6th movie the Empire was still a major threat that took decades to finish off and even then it was still around to mess with the New Republic. If you actually paid attention to lore you would know that. Even after SWTOR is over, the Sith Empire takes forever to go down till just Sidious is left and he turned the entire Republic into an Empire. So shut up. Sith always survive and they will continue to do so.