I think that if/when US goes under (which would be after selling assets) a lot of other major carriers (UA,NW,AA,CO,DL) would step in and pick up the most profitable assets (PHL
, 'Buses, regionals, EMB's) and start/add service to most of the cities US served. Because they are so East Coast centered, they are really flying in a very competive market; so there aren't a lot of cities that would lose all air service. But, it would make the market a bit smaller.
/// U N I T E D: It's Time to Fly.