Dulles is a key cut, I think for southwest.
Now there is no Dulles to Houston competition other than United.
Southwest could fly to Hobby and do pretty well, maybe. Although considering that Continental only flew 3 or 4 small planes a day; and obviously didn't make enough money to keep the service.
This will certainly degrade the quality of life in my office. I work exactly halfway between IAD/DCA, and we fly to Houston way too much. Out of 15 people in my section, on average one person is traveling to Texas every week. Most of my coworkers live closer to Dulles than to the office. Some very unhappy campers.
Maybe another midwest destination from Dulles will be added to Southwest's schedule ... that would be nice ...