Well, when we compare it to Halo 3, the best like for like entry in the series, the Halo franchise is definitely in decline. Halo 3 launched the last week of the month, like Halo 5. Both were the first main entry in the franchise for their gen. Both launched on a small user base, the XBO actually beating the 360 by 2M+ in the same time frame. Yet, Halo 5 did 3.3M just in the US, according to NPD. If these numbers are right, I doubt Halo 5 even did 3M WW. Probably closer to 2.5M.
It's definitely not a failure, but it just continues to illustrate how the XBO isn't doing any favors for Xbox franchises.