Halo Reach started this down spiral (the reduce in populations, massive changes to the game, 1-50 ranking system removal, etc.) and 343 has done a decent job of trying to bring things back. I actually enjoy playing Halo 4 matchmaking.
MLG peaked in the Halo 2/3 days and they didn't bother to pick up Halo Reach.
If Halo returns to the foundation that Halo 2 and 3 established (1-50 ranking system, split the playlists for ranked and social, keep a long-term XP system, etc.) then there will be plenty of people willing to try out Halo and will stick with it for a long time. Until then I'm content with Halo 4.