The increase in competition will indeed come as other carriers will be able to add service to DCA.
US can make the argument that small communities will lose air service and there is probably truth to it... but it doesn't change that in a slot-controlled airport w/ no meaningful way for carriers to grow, one carrier cannot control 75% of the flights.
The irony of all of this is that US swapped slots with DL at LGA, ended up with a smaller set of swapped slots at DCA than DL got at LGA, and will likely now have to divest a portion of the slots it swapped with DL. At the same time
Even though DL's share of slots at LGA is about 7% less than what US alone has at DCA, DL's share of seats at LGA is only about one percent lower meaning that DL is using its slots more efficiently at LGA than US is at DCA.
AA controls another 12% or so of DCA's seats.
That is the argument that regulators and competitors will make regarding the need for AA/US to divest.
One company could certainly argue that if they had all the slots available they could provide service to more cities but there is something about being able to use scarce resources efficiently and in doing so in such a way that other carriers can successfully compete.
The US industry is consolidating and regulators will not accept that B6 and WN will have such a smaller percent of the available slots and seats at DCA.
WN should indeed be able to pick up a few more slots but unlike in other cases, divestitures should be done in such a way to increase the amount of service to the most cities and not just shift slots from one carrier to another.
