2005 was when merger related expenses began to be felt. Prior to that AWA costs were among the lowest in the industry.
But this is what US Airways does - swallow smaller, lower-cost airlines (PSA, Piedmont). The airlines merge and presto! You still have a high cost airline. Except that now the territory once owned by the heretofore low-cost victim (California, the Southeast, now the Southwest) shares the burden of this same unsustainable industry-leading cost structure. So said airline is no longer able to compete in that territory, and inevitably cedes the region to a lower-cost competetor. In this case, it will be Southwest (again) that comes out the winner when US Airways (again) gives up the west coast.
It won't only be PIT pilots & F/A's commuting to PHL.