Frontier is a play on the mid to low consumer, simple as that. That's the segment that has been struggling mightly with inflation and job cuts of late, so not surprising their numbers are poor. If we still have another leg down to come into full recession then for sure they will be hammered even more. Still though I think there's a market for it at some point, lets see how they get to that point.
The delays in extending their calendar probably have been as much about projecting where the economy is going as much as figuring out how successful their modest pivot into competing on certain routes the majors have dominated in has gone. Really hard for anyone to gauge that without having access to their internal numbers but I can say my F9 flights this year have been 100% full, some even offering the credit vouchers to give up seats. And while still cheaper than the majors, they haven't been bargain flights. With the calendar ending in late May, its clearly time for them to figure out how to make hay in the summer and that might mean a temporary shift to getting the bargain shoppers onto planes for their vacations in lieu of chasing down some of the major's fortress revenues.