When Ghosts launched the initial numbers reported by VGChartz* they were surprisingly low. I figured Ghosts would move about the same number of copies as MW2 did on what I'm going to call last-gen hardware (360, PS3), and an additional small amount on current-gen. The numbers that came in initially had it doing significantly less, but that has turned around. The holiday sales numbers were about equal to MW2's, something I did not expect, and it sold quite a bit better than I thought it would on the new hardware.
With nine weeks of sales Ghosts has sold a total of 18.3 million copies while MW2 had 17.8 million sold in the same amount of time. I imagine that Ghosts will do substantially less business during 2014 than MW2 did during 2010, and it'll probably top out between 20 and 21 million copies while MW2 is working its way up to 25 million having sold 501,000 last year.
(For comparison Black Ops 2 sold 20.6 million copies in the first nine weeks.)
Call of Duty 11 this year will have an even bigger challenge than CoD10, especially if the rumors are true that Sledgehammer Games is taking the helm on their own. It will still be a cross-platform game, the console transition will still be under heavy load, going an extra year without Treyarch's Zombies mode will be trying for many stalwart CoD players, and even die-hard Call of Duty fans have a healthy amount of skepticism toward a new studio taking on such a large endeavor.
2015 is the year when I think we could see some seriously big changes in the game. It doesn't make sense to introduce a new engine until they're ready to release on next-gen only which I can't see happening this year. It remains the case that there are over 150 million 360's and PS3's plugged in and waiting for a game to play, and by November 4th there will at most be 30 million XB1s and PS4s plugged in ready to play.
For me CoD loses it's sheen just a little every year, but I still see myself getting the new one for at least two more years. We'll see how it goes.
* Many people say that VGChartz is useless but I disagree. They may not report accurate numbers, but they collect data for each game the same way which means if they are off it's by a similar amount for everything. That makes them somewhat reliable in determining the relative levels of sales across all games.