So while it had the best opening day sales, it was not the best selling Halo game ever. And the down side to having the player base drop so much is that when Halo 5 comes out we're going to be seeing massively smaller sales. Partly because there wont be as many people with Xbox Ones but also because so much of the user base will have lost interest.
I know I'm not buying Halo 5 on day one, if its anything like Halo 4 it will be 9 months before they've made it playable. At which point, I just wont care anymore.