With AA/US merger - the new AA becomes two-thirds of Terminal B/C at DCA. Term A extra. Lot's of choices for that one airline at DCA - which will make the new AA the dominate carrier at DCA. With AS partnership - that adds more long haul routes.

The plus for UA is the international flights out of IAD. But other than that - AA becomes the choice for DCA.

I keep my options open, but for now I am mostly flying AA (because of the much better upgrade rates). The AA/US merger strengthens that.

Just check out IAD on a Saturday morning - the place is a ghost town. More employees in UA terminal C/D than you see passengers walking around the terminal. With new long hauls out of DCA - I would say IAD is suffering for morning/mid-day passengers. Doesn't really get going into the evening.