I don't know how even that service would outweigh the advantages of DCA for domestic travelers. A suburban airport can't compete with a downtown airport if the downtown airport provides extensive services. LHR prospers rather than LCY because LCY doesn't offer a lot of services - if LCY offered as much as DCA did then LHR would also somewhat struggle in relation as far as short-haul travel is concerned.
IAD is an airport for people who need to travel overseas (such as myself) and I don't mind it as that. If you don't use UA and thus avoid the crappy concourse(s), it's actually quite pleasant.
The only way to make IAD a preferred domestic option would be to reduce the DCA service significantly. In Europe this likely would have been done already for environmental and noise reasons. But in DC's case it's a long shot given that the government types in Congress and the departments love the 10-15 minute cab ride to the airport.