There are 725,000 people in Denver. If we naively assume that one sixth of them pay for an internet subscription at $100/mo each, that's $145,000,000 annually that the people could use to fund their own ISP.
The people are held captive by a combination of forces that appear to have nothing to do with economics. If they could all communicate together they could easily fund their own alternative. Appallingly, the place that they are being held captive is a communications network!!