I’ve had the idea for a few years now that Delta or a conglomeration of airlines should invest in passenger rail from these small regional airports to the big hubs.
Phoenix has Sky Harbor in the city proper, and then Mesa Gateway only about 20 miles east. If they could just hook the light rail between the two, you could eliminate the need to fly in and out of the small airport.
In Michigan there are DTW, and then small airports in Lansing, Flint, and Grand Rapids. If Delta built rail along or above the freeways that connect those cities to Romulus (where DTW is), they could probably get rid of their service to those airports entirely.