I agree with some of what your saying
Halo 2 was basically the only thing to play, and was awesome, CoD 4 was the first BIG CoD, and im not sure if it came out before or after halo 3, but i do know they both came out close to each other, For me Halo 3 dominated, though CoD 4 was not far behind, With W@W coming out a year later, it basically had the same concept of CoD 4, but set in the WW2 era, so still, Halo 3 was still my preferred game, but i did play a lot of zombies, with mw2 coming out a year later, I couldnt help myself, it had so many new things, it just felt awesome earning a high killstreak, and getting multi kills, i basically let halo 3 collect dust, but after about 5 or 6 months of MW2, i grew sick of it, so i went back to my beloved halo 3, and enjoyed it up till the release of Halo reach… What halo reach came out, i didnt feel the same way when playing halo 3, it didnt have that lasting effect of Halo 3, I played reach up till the release of Black Ops, where the same thing happened, i grew sick of it, so instead of going back to Reach, I went back to MW2, because i couldnt enjoy reach like i enjoyed MW2, by this time i was pretty much a Call of Duty fan boy, when MW3 came out, i experienced so much dissapointment, after 4 or 5 months of MW3, i found myself lost, i picked up BF3, but that was not my type game, so i went back to Reach, and had a blast, these days im playing reach, i took my over a year and a half for me to enjoy it, but yes i love reach.
The point im making in this, is that the reason the Halo population is not as great as it once was is because these “one year games” , people look at games as being outdated after a year, this is all thanks to the Call of Duty Franchise.
I believe Halo 4 will be great, and think the population will be as strong as it once was… for a limited time though, Halo will never be the biggest game, because it doesnt release games annually
#rant