I think the downward trend in both public opinion of Halo and population began with ODST and Reach. Don't get me wrong, both are great games...but the changes were not as well received as Bungie would have liked. This combined with the fact that the Call of Duty explosion was really beginning in '09-'10 it all culminated into this weird beast and all the sudden Halo was not as cool to play as CoD (at least amongst the youth ie. the Target Demographic)

Then Halo 4 comes along...it keeps the changes that I and I believe a majority of the players were not fans of (Armor Abilites mostly) and expand VERY heavily on that system, creating an almost CoD like system.

To me personally, Halo is at its prime in the simplest form...everyone starts with the same guns, the same speed, the same abilities and it then becomes a battle of individual skill, team work and map control. Not who has the slight advantage based off of what silly perk, or ability they have.

Combine all of that with not necessarily franchise fatigue but console generation fatigue (8 years is just too goddamn long) and you have what we have today.

I think that with the next console generation and a return to the golden triangle rule we will see a reinvigorated fan base....also if someone walks around 343 studios and smacks artists for making silly changes. That would help too.