Aaah! I see what you got stuck up on then.
It's a game of statistics. Let's say there's 10 billion people in Heaven. Only one person sins and is thrown out every 100 sixtillion years (like some kind of half-life formula). Which means that 10 billion x 100 sixtillion years is what it would take for all to have sinned. Something like that. Infinity and eternity is a very long time, longer than any number, so if the chance is that someone sins ever so often, after a very, very, very long time, heaven could be faced with vacant lots.