Just as likely would be that the routes and aircraft would be divided up among some of the other lines. It isn't necessarily all or nothing. It could be done piecemeal. I certainly agree that a US/United alliance seems unlikely. It would be like lashing two rocks together and throwing them into the lake.
My point in saying that there's plenty to worry about was not that you should fret over tickets you hold, but that you should keep your eyes open and understand that USAirways is in very big trouble and may well not survive. The only thing you really have to worry about (unless you're a stockholder) is your frequent flyer account, and I think prudence would dictate bleeding that puppy down as soon as practical.