Town hall stated they are going to grow PHL,.Looks like they moved flights from JFK ,but if you say soI recall AA calling STL a "reliever hub" for which I never found to be viable solution, as I don't recall any such beast lasting long. How well did that work for UA in CLE or DL in CVG/MEM? Only time it has any chance of working would be if other hubs were at capacity, but with DL dehubbing DFW in 2005 and scores of gates became available, then STL as a hub would be redundant.
PHX might become that "reliever hub" as LAX would be slot restricted and smaller aircraft such as Express flights would connect passengers through the more available airport. PHL might play the same role with JFK for connecting passengers vs. high yielding O&D passengers.