The problem with Halo is not that it's a bad IP or that it has gotten stale. But it's no longer a titan and can no longer in and of itself sell a console. Maybe Halo 5 will be revolutionary (I'd venture to guess it won't be).
Halo 1 was a 10/10 game. Halo 2 and 3 were 9/10 games. Halo ODST, Halo Reach and Halo 4 were 8/10 games. The first 3 games each innovated quite a good bit, but after that, the series stopped innovating.
It was sad to notice that 5 years later, Halo 4, both online and off, is almost entirely the same as Halo 3.
My point is that Halo is still a strong IP, but it needs to exist alongside other strong IPs rather than being Microsoft's main selling point.