How quick Denverites are to criticize Denver International Airport, and then holler for more transatlantic/transpacific service.
I visit Denver quite often. (I was born there, and much of my extended family is there.) DIA is a terrific facility, much, MUCH better than Stapleton was. If DIA was not built, you wouldn't even have a chance at getting DEN-FRA service, not to mention DEN-NRT.
Where near Stapleton could one have built a 16000' runway? Nowhere, unless you wanted to use the Rocky Mountain Arsenal, and there are a legion of environmentalists and government officials ready to shoot you down there, not to mention the NIMBYs who live right there.
The real problem with fares in Denver are that people are used to paying them. United can charge what they want most of the time, and people will pay it. When United can charge exorbitant fares, others can too. Unfortunately, this is the way the market works -- businesses charge prices that the market will bear, and the DEN market is certainly paying out.