Spirit routinely highlights to their investors that they aren't taking market share from other airlines. They create new market share everywhere they go by making travel possible for people who simply wouldn't fly otherwise.

Some of their international flights are great examples. A person in the US with international family might be able to afford to visit once a year flying AA, but they can visit 4-5 times flying spirit. All they have to do is keep some clothes and stuff with their relatives so they don't need to pay a dime for bags, and have the forethought to bring a snack and (empty) water bottle with them instead of buying airport or airplane food and drinks. That $1000 AA seat costs $200 on Spirit.

Or some of the little airports Spirit flies to just a couple of times per week or just once a day. Those passengers would take a bus or drive at least partway to their destination if Spirit wasn't there. Their business model supports these low frequency pairings so they're pretty much printing money just by showing up with a $60 ticket as an alternative to taking the bus.

Last time I looked, Spirit was saying that they had identified over 600 additional pairings that would be profitable, and due to growth constraints they can only pick up a few dozen each year at most. That's 10 years of additional growth they already have lined up ready to go, at whatever profit margin they use as a cutoff.

Sustainable? When the economy burps or crashes again, where do their passengers go? Their fares are low enough that they think they'll actually be positioned better than anyone else since just about anyone will still be able to fly with their low fares. The primary threat in those situations is the 50% or so of their income coming from non-fare revenue. It remains to be seen if passengers will continue to pay $3 for a bottle of water during a recession, and that could cut into non-fare revenue side. They'd have to make up for that somehow, either cutting service on routes that don't keep buying water and snacks or whatever, or raise fares to make up the difference. In either scenario though, the fact is that they've got a dozen more seats in each plane than anyone else and their whole system is structured for low costs so their adjustments will look minor compared to everyone else who is still saddled with higher CASM.