The digital sales argument is tremendously weak. Yes digital sales have increased, but if they sold equal numbers digital and retail (no way) that would still leave them at 300k which is 70k less than Halo 3 did with a smaller install base. So yes, there is some significant shrinkage in that market, no matter how you look at it. These are just numbers, there's no emotion involved.
I'm not saying the franchise is dead, by any means. I do think the FPS genre as a whole is feeling the effects of buyer fatigue, though. The COD hype, for example, just seems to be fading exponentially every cycle. Beyond even the point of people complaining about them. Worse, now people are just used to them coming out.
I also wonder if releasing the collection so close to the launch of the next mainline entry might have contributed to fatigue instead of, as intended, hype. Especially with the technical challenges.