My recollection is that the existing DEN hub was expanded when Texas Air bought PeoplExpress - which had acquired the old Frontier, with its DEN hub.

The CO DEN hub was dismantled when the new airport opened - due primarily to much higher costs at the new airport.

Wouldn't surprise me at all if CO actually ran DC-10s to places like MIA in 1979-80 when The Shining was filmed. DC-10s were in widespread use on domestic routes back then.