That's not quite a fair argument. Reach didn't kill halo. COD did. An outside force. No matter how good Halo 4 could've been, there wasn't anything that was gonna stop black ops 2.there is a reason why h2 and h3 was the number one game played on live until the next title released. as soon as that garbage game reach came out with sprint there was a sharp decline in number of players. competitive or casual, NO one played H4.
Basically Other games really hadn't or just started to rise in popularity. Everything has its time. Halo had its time then it was COD. Now that's declining and the next game will eventually take its place( Will it be Destiny 2? Titans all 2...)