Parker wanted to run one of the big 3 and got it.
DL far succeed in what it got in the NW merger, even if some PMNW merger wish it had never happened.
US and DL's route system were very similar at the time of the merger while NW's was highly complementary to both DL and US. NW chose the airline that could create the biggest mass and have the best chance of succeeding and every indication is that the DL-NW merger, including the addition of Airbus aircraft and the greatly increased complexity of the fleet has worked beautifully.
Let's get back to AA-US in five years and see where they are at the same point. They may or may not have reached the same point or even gone past. It is simply too early to know.
I don't think anyone in leadership, on the board, or stockholders at DL have any regrets about the decision to spurn US and go with NW>