Black Ops is still selling well, though. It must be, because it is priced $49.99 on Amazon which is only $10 less than it was when it launched a year ago. If sales had slackened the price would have been reduced to $19.99 or something, which is how it goes with most games. Plus it was only added to PSN as a downloadable title only a month ago and it is priced $59.99 there. Someone must be buying it at that price or else they would mark it down.
The Modern Warfare series of games are sorta like Madden in that the newest one is almost certainly the best one in the series, and the only one worth owning. But the Modern Warfare games don't come out every year, they come out every other year, and in between those releases are the Treyarch COD games. I think this helps explain how the series is able to avoid the stagnation of Guitar Hero, because here you have two studios working on different lines of games within the franchise. MW3 isn't significantly different from MW2, but it is significantly different from Black Ops. Someone whose first COD experience was Black Ops (and that would be a lot of people, apparently) would go on to MW3 and would notice a dramatic difference and think its a worthy change to justify the purchase, but that would not have been the case if MW2 was their first game, and then next year MW3 came out and they went straight to that without Black Ops in between. So these two studios alternating back and forth with the franchise helps keep it somewhat fresher than something like Madden or Guitar Hero.
Since this year was an Infinity Ward game, I would expect next year's COD to be one made by Treyarch. Will it be Black Ops 2? Will it be World at War 2? or will it be something completely new? As long as it isn't going to be MW4 I'll be satisfied.