Well, not exactly zero-sum in the game sense, but I notice that my reputation goes down when a user closes his account.
I found this question, which implies (but doesn't quite state outright) that points are always removed when user accounts are closed. But it seems to me that this means that eventually everyone's reputation will go to zero (if they don't cancel their accounts first), since eventually all users will die, if nothing else.
Is this the way it works? Is this the way it's intended to work?