This, combined with gauge. The NK model, like the F9 one, is based on volume - getting as many seats onto the plane (and pax) as possible and then competing directly against full fare carriers to peel off some of that traffic. Basic economy gave you the same thing, but on an airline that offered multiple flights a day. The ULCCs that are doing relatively well are those that either 1) Focused on lower gauge/demand markets (Breeze) or secondary airports (Allegiant). Avelo was set up to replicate the Allegiant model (as they were all former G4 guys who started it), but Breeze found that they could beat them in their own markets with the lower per seat operating cost aircraft in the A220.
The secondary airport thing is already getting real again. I just got Breeze to enter the LNK market because I showed them how much they were going to have to tack onto each ticket to pay for OMA's $1B terminal if they chose to fly there. They can operate for less at LNK, and it turns out that 8% of their pax right now are coming in from Omaha.