At first I thought Halo 5 sold horribly, which is true if you just compare it to its predecessors and take nothing else into account. But as you said, looking at sales of other games it actually didn't do so terribly.
Halo has held up better than the two other major yearly(which is what Halo has become) franchises from last gen.
2012 360 vs 2015 XB1 sales
Halo 4 - 3,662,557
Halo 5 - 1,457,849
60% drop in sales
Assassins Creed III - 1,568,017
Assassins Creed Syndicate - 257,789
83% drop in sales
Black Ops 2 = 4,558,055 (US+UK+GER)
Black Ops 3 = 1,690,482 (US+UK+GER)
63% drop in sales
Halo is certainly down from it's peak in the Halo 2/Halo 3 days, but this decline happened a long time ago with Halo: Reach, whose first week sales were lower than Halo 3 despite a 4x increase in install base. Halo 5 sales reflect that decline without the massive install base difference that allowed Halo Reach and Halo 4 to stay close to Halo 3. If sales for Halo 6/7 continue to improve as the install base increases, I think Halo is in a decent spot. If sales stagnate at the 1.5mil first week mark, then this decline is a big problem for Microsoft as their biggest exclusive becomes just a mid-tier title.