The area has enough traffic to support three airports, and all three (DCA/IAD/BWI) carry between 26-27 million passengers a year, each. I don't think you could close one of them without some significant disruption to service.

Travel in/out of IAD from DC can take an hour, which is obviously why people there prefer DCA. And the flights there are all short-haul anyway, so many are the types of flights people are doing on short turnarounds.