
Halo Wars 2 did not sell many units, we can be very sure of that, but it stands to reason that those fewer customers bought more stuff in game, whether it was card packs, the season pass/leaders, or ATN. (it SHOULD be the only source of revenue, but that's an argument the rest of the internet it already screaming about) So even though Halo 5 almost certainly did not outsell most other Halo games in terms of units sold it almost certainly made more money per player than any Halo game to date, and that's important. Simply put, copies sold isn't exactly the same barometer it used to be simply because it's no longer the sole source of revenue. I forget the exact quote off the top of my head) Note that they very specifically did not say "most copies ever sold." In fact they haven't said a word about copies sold for almost any game this generation. Keep in mind that after Halo 5 launched MS's big thing they liked to quote was that it had the highest revenue of any Halo game ever. I'm not so sure lower than expected sales is the deterrent to a sequel you might think.
