World of Warcraft did it with its absolutely massive amounts of endgame content it built up over its long lifespan.
Star Wars: The Old Republic failed at it because it had none of said end-game content.
EVE Online survives because of its incredibly complex economy and high risk factor.
So, what does SK have to keep people from leaving? And what could be added to keep people from leaving from boredom?
Event after Event after Event after Event.