It's not necessary for every single flight to have multiple acceptable flight options for the market forces ("vote with your wallet") to have an effect. You could not go. You could drive. You could fly into another airport and drive the last leg. You could charter.

I go out of my way to choose Delta(/KLM/AF), JetBlue, and Southwest over United, Frontier, Spirit, and Ryan. That works, even if it means I once in a great while fly on United because that's the best choice for that particular trip/leg.

How many markets are served by, say, United and only United? Other than their Micronesia operation, I can't think of any where you wouldn't be able to drive to an alternate airport within an hour or so drive (really just EAS airports).