Southwest leadership has said that the plan is to grow denver to 250 flights, maybe as much as 300, and have it be their largest station. United has called for a 50% capacity increase at DEN.
Combined with the growth of F9 at Denver, I don't really know if I can see that as anywhere near being sustainable.
From the DIA website:
"This ranks DEN as the third-fastest growing airport in the world, behind only Kuala Lumpur and Delhi
DEN as the sixth busiest airport in the U.S. and 18th busiest in the world (2016)
Approximately 65 percent of travelers at DEN are origination and destination (O&D) passengers, and 35 percent are connecting
Approximately 35 million annual domestic passengers are O&D passengers, making DEN the fourth-largest domestic O&D hub in the U.S.
International traffic at DEN accounts for approximately 2.2 million passengers annually – approximately 4 percent of the airport’s total passenger traffic"
'DEN is United Airlines’ fourth largest hub by capacity, serving nearly 140 destinations from DEN
United makes up just over 40 percent of DEN’s total seat market share
60 percent of United’s traffic at DEN is made up of connecting/transfer passengers
DEN is also the fourth largest hub in Southwest Airlines’ network by flights and capacity
Southwest serves over 60 destinations from DEN and makes up 30 percent of DEN’s seat market share
About three-fourths of Southwest’s traffic at DEN is O&D traffic
The Airport is also the largest base for Frontier Airlines with over 50 destinations from DEN
Frontier makes up over 11 percent of DEN’s total seat market share
Frontier’s traffic is nearly 90 percent O&D traffic at DEN
DEN is also served by American Airlines, with seven destinations from DEN, as well as Delta Air Lines, with nine destinations"
If UA added enough capacity to boost from 40% to 60% (i understand that this is now represents 120%), that would push DEN to 72m pax per year. WN sees 200 flights/day at DEN already, so if we assume that an increase to 300 flights represents a proportional increase in passenger count from the existing 30 percent share then that each 50 flights per day that WN operates out of DEN is 7.5% of the airport's existing traffic. So then from UA and WN alone, proposed increases in traffic would escalate to 81 million passengers per year. And that's not including frontier's recent rapid growth.
Something has to give. DEN is definitely a strong aviation market with the fourth highest number of O/D pax per year as it is, but I honestly don't see how a metro of 4 million people can support an airport with 80m pax per year. There isn't any major extraneous factor that would let DEN support 80m pax / year. I don't think that DEN is going to become ATL 2.0 either... each carrier faces so much competition as it is. Delta nailed ATL because they have kept competition at bay for decades. But DEN has hub carriers with market shares of 40%, 30%, and 11%. I don't think that DEN is conducive to becoming a fortress hub for any carrier, because as UA expands, WN and F9 will eat into their revenues, and a megahub doesn't really fit within F9 or WN's business model.
Washington-Baltimore-Arlington is a high income, high prestige metro area with 9.5m residents and roughly 60m pax/year at its main airports (baltimore washington, reagan national, dulles). Denver currently has 3.5m residents with 60m pax/year. DEN is obviously more conducive to a stronger aviation market but I think a lot of the growth recently has been to catch up with demand and to compete with competition. That's why fares are staying low. I genuinely don't believe that DEN will support 80m pax / year as WN and UA have proposed.