Neither AA nor ORD are going to invest in the terminal too much while the overall airport modernization program is underway. That said, while the airport is owned by the city, the airlines have considerable influence on the look and feel of their terminals. If AA really wanted to modernize their terminals at ORD they would do so. DL is spending over $1B on modernizing their terminals at LAX.
For the haters who are going to keep hating on PHL I have a few thoughts for you: EWR, MCI, FLL. Those are legitimately horrible airports. PHL is not the best by any means but it's far from the worst out there.