Its obviously losing steam, similar to World of Warcraft. Despite being different genres for totally different demographics, both games were at their peak a few years ago, but bleeding fans/subscribers on a monthly basis.
There is a sense of closure for Halo as a whole, Bungie finished the trilogy, made a prequel in Reach which is always a sign that something has been milked dry. Then Bungie left, which in and out of itself is the "ending" of any franchise. 343 had a chance to put their own spin on it, their own take, a start of a new trilogy, but ultimately just made Halo inspired by CoD.
Instead of being a leader, Halo is today a follower, its no longer even the second biggest FPS on the market, that belongs to CoD and Battlefield. And who knows how big Titanfall and Destiny will be.
It sucks because i love Halo more than CoD or BF, and Titanfall/Destiny will have to be pretty amazing to top Halo in its prime.