Oh for gods sake people, they killed Coulson off in the middle of the movie, it was a major plot point. There was nothing vague about it. Huge dagger through the chest and he sat dead with his eyes open whilst fury looked on, not running for a paramedic.
This is all post rationalisation because they decided they needed him for a spin off TV show. Nothing more.
It's comic book land, they can do what they want really, but rewatching the Avengers after the Shield TV show comes out with a living Coulson in it is going to be a more heartless experience.
His death gave The Avengers the emotional depth that it needed, unexpected and moving in the middle of an action fest. Personally as much as I love Coulson as a character, I think it's a big mistake.
But there is money to be made and money talks, they have already cashed up on The Avengers.