Yeah, multiplayer seems awesome but with such a huge team they just have to manage to get a campaign together which takes some more hours, is playable with friends who don't have to sit on another console like many many Halo fans did over a decade and they have to listen to what those who will buy the game want and if most say Master Chief is for Halo like Lara for Tomb Raider or whatever then they have to accept it.
You can change playable characters in Battlefield, Killzone and games like that but when you have a game where most simply want a certain character to play then you should listen to them. Don't change the main playable characters in Halo, Tomb Raider, Zelda...this is just a very bad idea, at least for the main games in these franchises.
There is a reason why you see so many cosplayers as Master Chief but not as specific Killzone character, because people care much about the Chief but not about Killzone characters^^
