The number of lines and PHL trips is not pegged to the amount of departures and block hours we have out of PHL. PHL trips can flow into other hubs just as CLT and DCA trips can flow into PHL.

I doubt PHL will become senior. The opening of PHL was a smart move for recruitment. It's strategic to keep PHL junior by increasing the lines and trips based on the pilot demand. If it gets senior, more lines and vacancies can be added to bring it back towards the junior side. DCA is proof of this model, it's not a big base but large enough to stay junior for captains and FOs.

For new comer planning purposes, I would assume you can get PHL shortly after training and hold it as a junior CA.