The reality is that American domestic competition is varied and spiky. The legacy cartel is careful to respect each other's turf. Each one has its cozy fortress hubs. Southwest used to be the great agitator until about 10 years ago, when they slowed their capacity growth and now mostly play nice with the legacies. That leaves the LCCs to strategically compete on select routes, giving America a mix of highly competitive routes and monopoly/duopoly pricing on other routes.

It's been interesting to watch the "cheap" and "expensive" routes out of my own airport (MCI) evolve significantly over time as LCCs start and fail and as WN decides to add or reduce service.