But they always do. Greed and lust for power are Sith attributes. They wouldn't want someone without the greed for power, without the lust for power, without hte ambition to kill their master as a Sith to start with. Luke would still have Obi-wan, Yoda, Anakin and potentially Qui-Gon to instruct him after RotJ, or at least guide him. And I'm sure some Sith wrote something down out there. Night Sisters aren't EU anymore. I think they'd make great villains in the ST. They were also in hiding. Easier to hide 2, sometimes 3 individuals then 400.