I'm not sure this is true. The spirit bankruptcy is a proof point that BE has been an effective response to ULCC competition. Allegiant, Avelo, etc are all playing a different game. With WN looking more and more like a legacy, and supply of new aircraft limited, I can see a more sustained oligopoly emerging, the conditions for which haven't exited previously. Not a foregone conclusion, and certainly not because they are doing anything premium, just a question of market power.