I don't agree very strongly with MacGyver's argument. Halo has competitors which are still able to sustain rank reasonably well over long periods of time, and it's not like Halo 4's dropping off is blatantly correlated with the end of DLC releases; it's been dropping down since the beginning, with DLC releases and updates occasionally giving it a momentary boost.
At best it's a partial explaination, and one which some other games fighting over the same space seem to be less subject to the effects of.