Thomas, it's pretty certain that US Airways is going under. Their CSM's are stratospheric, even compared to the other 5 Cartel carriers, and the low-fares are eating their lunch. Plus their unions are in no mood to make work-rule or cost concessions. And management wasted a year of potential repositioning by chasing after ultimately-unwilling sugar daddy UA.
After US goes under, I'd bet WN will be there before they get the last US plane shipped off to Marana. PIT people are so used to sky-high fares on US (heck, they couldn't even bring themselves to flee to AirTran's excellent service and low fares to PHL and LGA), that the trauma of losing US will shake them out of their hypnosis. WN will clean up in PIT. No other Cartel carrier is going to start a hub there because PIT doesn't fit their plans and doesn't offer enough international traffic potential (except possibly CO, see below).
I'd look also for PIT-BUF and PIT-ALB routes; US now has the only nonstops to Upstate NY. Also maybe AirTran would introduce ROC-PIT. Speaking of which, look too for AirTran to re-establish a focus city there once US is gone. There are lots of cities out there that WN doesn't serve--like PHL and LGA. This time maybe Pittsburgh business people, and their corporate travel departments, will be receptive to lower-fare service on these routes.
I still wouldn't rule out CO moving from CLE. PIT is a vastly superior airport facility, and CLE with its allegedly larger O & D base can only support one 752 to LGW. PIT has been supporting 762's, and before Sept. 11, A330, on the route. Plus 762's to CDG and FRA.
In which case, WN would still move in, but I don't think they, AirTran, or anyone else, would grow nearly as rapidly or extensively.
Need a new airline paint scheme? Better call Saul! (Bass that is)