I've started to play Halo with H4, so you, old gamer, can disagree. I loved the game, the most original FPS I have ever seen, but I noticed that there are few players for a Halo game in it's first year. H3 was the most played game in live for much time, but H4 isn't close of the players quantity it's predecessor. After think a little, I understood that CoD wasn't a franchise so strong as today.
There was a vacuum between 2007 and 2009, without Halo, then CoD had it's oportunity to take the throne. I mean that H4 is not the real problem, it can have had some alterations that displeased the old school fans, but is far to be a bad game. The truth is: CoD has conquered hardcore and casual players, including some Halo fans, the public is in love for military games, rapid reflexes based. Is how the Halo time has passed away.
Lets see what will be the "CoD killer".
It's just an opinion, sorry for the english not so good, my native tongue has many prepositions, then sometimes I don't know the right time to not use them.