People who live in cities, who are an increasing portion of the population. Large hubs tend to be better connected to public transit than their smaller counterparts. In most cases I'm happy to pay a premium to fly out of DCA compared to IAD, with the former being located much closer to the city. In many cases I'm also willing to take a one-stop ex-DCA than a nonstop ex-IAD. That will change slightly once IAD is properly connected to the silver line, but DCA will remain substantially closer. As we become increasing urbanized, the viability of secondary airports will be increasingly tied to their accessibility via public transit.

If Moxy succeeds, it's going to be because they're flying comfortable A220s that offer a nice onboard experience with comfortable seating and plenty of space in overhead bins.