I think the difference is that there is one less ULCC to compete with. To add NK to the mix brings on the same number of ULCC seats and a debt mess that F9 takes on with very slim margins. For F9 to stay away from NK allows F9 to grow at a much more measured rate with less competition and debt. It enables F9 to get their finances in order and become steadily profitable. The legacies have hurt the ULCC model with basic economy. My guess is that F9 will walk away from NK because they can do better without them.