flights out of IAD. You can fly IAD to virtually every large city out west (multiple times per day) on United. DCA has something like 2 dozen slots for perimeter-excepted flights, and each airline has a handful.

What would really kill IAD would be a customs facility at DCA and international flights. As the new 737max and 320neo series aircraft come online, they're able to both 1) operate in and out of DCA's runways and gates and 2) operate flights as far as Western Europe. Especially if airlines could set it up like BA used to do at LCY for their A318 Baby Bus flights to New York. It was almost like flying private with dedicated check in, security and gate lounge -- you could be in your office at 5pm in DC and on a 6pm flight to London out of DCA.