Two points:
1. CLT doesn't have the O&D, just like PIT. If WN ever decides to enter they may have a profound affect, just like BWI.
Charlotte: cheap to operate out of
Pittsburgh: Expensive to operate out of
Charlotte: Is not near another US Airways Hub
Pittsburgh: Was near another US Airways hub
Charlotte: Provided US Airways access to the southern region
Pittsburgh: Provided US Airways access the same region as Philadelphia
Charlotte: Is in a region where there are only 3 airline hubs (Airtran, Delta, US airways)
Pittsburgh: Is in a region where there are 8 airline hubs (LCC, CAL, UAL, B6, DAL, AMR, WN, Airtran)
What makes you so sure Southwest would tear up US Airways at Charlotte? Surely you do not come to these conclusions based on US Airways pull-out of Pittsburgh? If you think US Airways would dismantle Charlotte just so Southwest could have flights to Las Vegas, Phoenix, Baltimore, Philadelphia, Orlando, Tampa, and Chicago (the cities that Southwest flies from Pittsburgh) ... then... power to you.
US Airways is strong at Charlotte - Unlike in Pittsburgh where US Airways was weak. If Southwest only has 7 destinations in Pittsburgh where US Airways is clearly not willing to put up a fight, what makes you think Southwest would come in Charlotte and decimate a very strong US Airways?
Edit: Regarding your second point about Shutting out IAD or PHL in a possible United/US merger. Neither would be shuttered. Both have large enough O&D to warrant hub/focus city status. Unlike Pittsburgh.