Can't see MAN-PHL being dropped any time soon.
The route is far more reliant on the Florida and California/Arizona market. Biz gets filled because it's price competitive against the likes of BA and Virgin.
Seeing as US is heading the merger with AA, I can't see why they would want to meddle with one of their own successful routes; MAN does well to be a European "regional" airport attracting the largest aircraft in the US Airways portfolio, and it's little to do with the patronage of Astra-Zeneca.