While some people leave, maybe 10 - 15 of your fc that most people notice, another group comes into the game. Some of those people will leave, some will stay, thus the nature of an mmo. When you find your particular taste of a game, you stick with it. Every game has forums threads about people leaving. Last game I played for 5 years, in that 5 years tons of threads of planning to quit and people deserting that game type apocalyptic threads, but the game is still played and running 5 years later just the same.