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.
Sure, but to retcon it like that takes away from the emotional resonance of that announcement in the film. I hate how death is so mutable in comics and I hoped it wouldn't happen on the TV show. Perhaps Coulson's popularity trumps the poignancy of his death announcement, but I don't think the guy is needed to do a SHIELD show. Coulson's part was well played and the character was well used to connect the dots. But those dots are connected now; Coulson served his purpose and 'died' with honor in the Avengers. That was enough for me.
Plus, retconning it along those lines just make Nick Fury extra-douchey when just saying he was injured and hanging on to life instead could've had nearly the desired effect of rallying the Avengers.