One of the hijacked planes came from IAD, though. A hijacker would not have enough time to seize a DCA departure before it was past the "prime" targets. I don't see how DCA's closure would change things; it would be another hollow gesture not unlike the towing of 2500+ cars at BOS. But the bottom line is that Congress will not put up with the inconvenience of flying out of IAD, so DCA will stay open.