A lot of people forget this HUGE piece to the puzzle. There is a place for a ULCC IF there is no competition for that slice of the market. For a while, Spirit was the only player in that market and they had it all to themselves. When basic economy was rolled out at the legacies and other airlines adopted the model, Spirit got massively diluted and had to cut fares further to retain market share, often at a loss. This is when the margins started compressing pre-COVID. Then Covid hit, and everything went tits up.