It's not always about the sales trial star. It's about the longevity of a series. Continue to sell out your games, and like that dirty hooker down the street, people will stop coming by. Your original fanbase will disappear, the people who supported you from the get go and a casual audience's allegiences can change mighty quickly. Creating a game to last and that is really fun and great will make sure that when you gain fans, you don't lose any either. Each year, COD loses many fans, but gains many too. There can only be so many "new" fans before the well is tapped fully.
Don't forget Activision's track history. If they were able to pump out 10 CoD titles in two years like they did with Guitar Hero, they will do it, but they need more resources for this game and some thinking when doing Multiplayer and some new feature to draw in sales.
This game lasts longer because it is annual releases and not monthly or every quarter of a year.