I don't disagree, but having it as a prequel makes coming up with some fantastical excuse for Coulson headlining it moot.
There doesn't have to be a fantastical excuse. People in real life have survived comparable impalement injuries if they got medical treatment in time. And we didn't actually see Coulson's death; it was announced off-camera by Fury, a character that we know for a fact was willing to lie in order to manipulate the Avengers' emotions. So there could be a perfectly naturalistic explanation for Coulson's survival, the seeds for which were already planted in the film itself.