There are an awful lot of factors at play, not a simple answer. The PW engines was a major headwind, however larger economic factors were a much bigger problem.

I left Spirit about 8 years ago, and here's my take on one of the things that is killing Spirit. TLDR: Spirit's customers quit flying and many factors including the engines kept the company from adjusting to the new marketplace.

When I was there, every quarter and every year Spirit would update their investor presentation with slides showing the sources of current and future revenue. Spirit would highlight that they could make money on a LOT of low frequency city pairings that no other airline could touch, because the target customers simply couldn't or wouldn't fly on other airlines. The year I left they were saying they had over 500 low frequency (2-3 times per week) pairings that they could make money on, but could only add maybe 20 per year if growth remained strong. The examples they used were in many cases low income families that were separated from extended family members, both within and outside the US. A family that might have $400 free income could visit family 1 time per year on a legacy airline, or 4+ times on Spirit using an $89 ticket and no baggage because they could leave clothes and stuff with the extended family. It was a pretty big market 8 years ago, lots of people took advantage of the ultra low cost no frills travel Spirit offered. Then a few things happened.

The engine issues put severe constraints on both Spirit's ability to expand into new markets and new city pairs, and even started to put a huge crimp into existing operations as overhead expenses for parked aircraft soared. Remember, back then Spirit was proud that not only did they have a very consistent fleet configuration (all planes had rafts, good wx radar, etc), they also owned outright a huge percentage of their fleet instead of just leasing. So when they parked them, all the ownership overhead expenses came directly from spirit itself. Then the economy changed, in part due to politics but largely due to covid. Covid plus political influence imbalanced the overall economy and inflation soared. Unemployment went way up, then way down, then whipsawed back up, while wages for lower income workers stagnated. That blew up the finances for a big part of spirit's customer base. That family that could barely afford to visit family pre-covid is now working an extra job just to pay for groceries. Suddenly all those low frequency pairings aren't consistently profitable. Spirit needed to rapidly adjust to the new economic reality, and guess correctly on which way to go. Double down and ride it out, or pre-emptively lay off half the company and retrench into markets where they still had customers. Without a crystal ball, which way should they have gone? They placed their bets, and no matter what anyone's opinion was then or is now, it's obvious now looking back that the changes weren't enough and/or were not in the right direction.

Yes, I only had a year and a half with Spirit when things were going GREAT, and my view on this is as an outsider. But I've been trying to pay attention because SWA was only a half-step behind Spirit on the road to ruin. SWA had the resources and customers to ride it out, and still ended up with a semi-hostile board takeover that forced some changes to happen immediately instead of gradually. Time will tell if any of that helped. Unfortunately for Spirit, riding out the combination of negative factors just didn't work because none of the negative factors changed in time for the company to survive. Maybe radical restructuring earlier on might have helped, maybe not. What we know is that what actually happened didn't work out. It's a shame, because Spirit really seemed to be trying to do things "right". Cut costs where possible, invest a little more where it helps long term. I really appreciated the company's efforts to keep a consistent aircraft configuration and low to zero MEL count, as it really smoothed out day to day operations. I don't remember many unscheduled plane swaps for MX or issues with unacceptable configurations (like not having rafts to get offshore), so the company's efforts to invest in the aircraft configuration and maintenance really helped out. It wasn't cheap though, and didn't help enough when the other factors went bad.

Another ULCC I occasionally jumpseated on, you know the one with the critters on the tail, pinched pennies in a different way. The last JS I did on that brand, they were carrying 8 MELs in a bare-bones airframe without winglets, a crappy radar, and no rafts. They survived, even though 8 years ago the fun rumors were about Spirit buying them, not the other way around.