Having literally billions of dollars in hardware optimized for flying that IATA says isn’t going to rebound for a few years is a huge overhead cost, as are multi-type fleets in general. And the A321 NEO, it doesn’t matter who is flying it - is going to severely impact the WB monopoly on some of the shorter international routes. And while the legacies can, have, and will use their wide bodies on heavily traveled longer hub to hub domestic routes the CASM will never really be competitive with aircraft optimized for those routes. And the higher percentage of more fuel efficient NEOs in the ULCC fleets of F9 and NK are going to mean they are less effected by higher fuel costs.

If the ULCCs can’t outhustle the legacies in this environment I’ll be greatly surprised.