It's definately being milked, and eventually sales will suffer. This has basically meant they've been gradually going downhill since CoD4. People say it'll last because they bring in changes to the setting, the MP aspects etc. But let's face it, the game mechanics and level designs haven't changed significantly since moving out of World War 2 and that has more to do with the new weaponry available forcing different playstyles and levels than the setting change.
At the end of the day, the core gameplay (which is the most important factor) remains the same with every iteration. The comparison with sports games is also false, because sports games have updated rosters which is why people buy them, even if they're the same game. Even then, without some innovation, sales do dwindle. They can't keep bringing out the same game with new coats of paint. Eventually they'll have to innovate and that is difficult when a new game is being released every year.
