From the mouth of Scott Gimple on Talking Dead.
Question was: You did not reveal who Negan attacks at the end so why was that the decision?
"The end of the story is what people saw. When we reveal who was on the receiving end there, thats going to be the start of another story. The kickback effects from that, what it makes everyone into, how people react, how the world changes for everyone, That's the next part of the story."
Doesn't begin to change my opinion on it.
Also Robert Kirkman says people should be trying to figure out who it was because "there were some hints there."