Spirit, Frontier, and all other ULCCs were born based on using predatory pricing against the big 3. And they were extremely successful.
To combat the ULCCs, Delta came up with Song and United launched Ted - alter ego LCCs within each airline. Both were resounding failures.
At some point in 2012, Delta launched Basic Economy to compete on a limited basis with Spirit and Frontier. It was marginally successful. Then in 2016/7, both United and American launched Basic Economy. All 3 airlines returned the favor with predatory pricing against the ULCCs.
And Spirit hasn't turned a profit since 2019. The same with Frontier, except they finally turned a small profit in 2024. That won't last.
The ULCC model is dead, killed by Basic Economy.

What the ULCCs did was cause all airlines to lose money on passenger tickets. The reason why the big 3 are able to make a profit is their credit card programs. If not for that, they'd also be losing money.

The entire industry has changed and Spirit cannot reinvent itself enough to be profitable. The big 3 will price their Basic Economy product with both seats and price points where Spirit and Frontier (and JetBlue and Southwest) die a death of 1000 cuts. Spirit and Frontier don't have lucrative enough cargo and credit card contracts to make up for the passenger losses. (I can remember working a midwinter flight LAX-LHR on the 777 where we had a mid-teens pax load. I asked how much money we were losing on the flight - we weren't; the belly was full of profitable cargo).
This is a brutal business with razor thin margins. And right now, passenger margins are negative for almost every airline including the big 3.
Can Spirit suvive? I suppose if they stop flying to major cities and go with an Allegiant, Avelo or Breeze model of servicing secondary cities. As a much, much smaller airline. If they don't run out of money first.