People leave games for many reasons. I have had to leave games before because of RL issues, PC issues, Internet connectivity issues (I hate AT&T Broadband)
Some people leave the game because it becomes boring, they are in a bad way in the game, they become frustrated about something, or simply loose interest in the game.
If you say you going to be in for the long haul, most stay. I understand that there is a list of hard core players that often play, and seldom drop out. Those people are the ones you want to have in your game. That is if you enjoy getting your arse handed to you on a silver platter as your people become slaves.
