Those are the launch numbers. Across both Xbox and 360. I don't agree with your perspective.
What you see as a lack of growth for Halo, I see as a extremely solid and loyal fan base that existed even on the original Xbox (it also sold >3 mil also).
I'm sure MS saw a decline coming with Halo. But I just don't see them forseeing a 70% decline.
Much of that "loyal fanbase" didn't have or want the console required to play the game. Especially after MCC was broken for over a year.
And that 3 mil in a month was never just core fans, it was casual and core fans alike.
Edit: what you see with h5 launch is what happens when only your remaining core fans show up after many are lost to Sony and MCC...casual fans have many more popular options