Terenz predictions may be correct. This is anyone's guess. One of many scenarios may include:
2003: sale of IAD hub to DL or AA. UA depends on US for Eastern Seaboard travel.

2004: DEN closes. Chance of NW or CO buying it but I wouldn't count on it. MIA closes, DL picks up routes for free after UA can't sell it. Either that or Air Tran bids and DL ends up with the routes.

2005: SFO and Pacific routes sold to AA. UA becomes a ORD only airline.

2007: UA shuts down. CO opens a ORD hub and in a promotion, offers passengers a free 10% of their Mileage Plus balance to One Pass. AA also offers 10% to match. Passengers lose 80%