PHL, regardless of US presence, still suffers from one thing that no airline can cure: location. Being stuck in between the NYC and DC air corrdiors equals getting sloppy seconds when it comes to ATC preferences and delays. WN mitigates that a little by having the relationship with ATC that they do, which I've heard is a very good one that has mutual benefit.

Regarding CLT, I think it's a foregone conclusion that only WN currently has the cojones (and the financials) to go into a competitor's fortress hub and embarrass them. CLT is a matter of time, because US will run out of money with which to fight.