Skyrim fans are too smart to pay $60 for the same game that came out last november, in a different box.
Actually its more of a question of addiction then intelligence. You see when the new game comes out the old ones servers inevitability become a ghost town. It might not happen immediately. But in a span of several months, if not sooner, they're won't be a whole lot of people to play the older game with online.
Indeed the companies might "encourage" this migration by shutting down servers for the older games. I wouldn't put it past Blizzard Activision to do just that, and since they run the servers, there's very little there customers can do about it.
So the fps players gotta buy the next game if they want to keep playing with there friends.
They don't have to change the product much, cause they really don't have to. It's almost like selling cigarettes, your customers are hooked.
I actually like playing a few bouts of BFBC 2, but I know that the game is destined to have short term replayability, especially when Battlefield 3 comes out.