I really don't think MS expect Halo numbers to have such a huge drop .
For gears they had Judgement numbers so it can't be a shock .
Halo on the other hand had over a 50% drop at retail if i remember right .
I just don't see anything leading up to halo5's launch that would have convinced them it would be nearly as successful as halos most successful launch.
It didn't have the installed base of H4. Didn't have the allure of Bungie's swan song in reach. It didn't have the hype or marketing of h3.
On the flip side, it was coming on the heels of two 343 games that massively disappointed fans, at a time when Xbox was bleeding former customers (including previous halo players) to Sony. Not to mention, numerous high profile titles had been seeming declines.
If they have 1 market analyst on the payroll, they expected a drop. Probably why MTs are so ingrained tbh.
If they want halo to do numbers, it needs a formula shake up that captures what the kids are into these days... probably a shared-world game. The sandbox is actually perfect for it.