They can, but the less people stick with COD4, the more fans of the franchise will be forced to move on to more profitable endavours for the franchise. Buy new game, buy new DLC, buy new maps, buy new guns, pay monthly, whatever it takes... As long as they can take you for every dime you have, but there are to few people actually willing to stand up to this crap at one time, so they can continue this kind of behaviour. Confront them with it, they will tell you they need to maximize their profits to attract investors and to keep the company going, cause just making money does not cut it, you need to make loads of it....
It always goes comes down to stakeholders making money, as long as they can cut costs/increase revenue, the companies hands are as tied as ours... Untill consumers stop buying, this will happen over, and over, and over