it's certainly not going to become unprofitable any time soon. Halo is almost too distinct in style to earn permanent perennial status. On the other hand, Call of Duty is just "real" war where you play as 1st world military powers, which will never not appeal to gamers. That gives it a pretty permanent feeling, like a sports franchise. They would have to take in a 180 direction for a few years to sour people on it.