To be fair, Spirit suffered from multiple reasons and a merger with Frontier may have just delayed the inevitable.
Note: I was very emotionally invested in Spirit because they made possible many fascinating trips that wouldn't have happened so my take may be biased. I can only think of a handful of companies that clearly positively impacted my life and Spirit was one of them.
1. Post COVID, the structural costs went up: pilots, flight attendants, suppliers demanded more money. Spirit paid decently well, had decent benefits from what I heard. When you are Spirit...to attract good people you gotta pay. It really showed in their operations, they had 0 crashes during their 34 years and had notable excellence in their maintenace operations. People thought they flew rickety junk planes. The reality was that they had a stellar run.
2. Pratt & Whitney GTF issues: Spirit actually had quite a young Airbus only fleet. Great for lowering costs through fuel/maintenance savings, customers enjoy the newest best planes...the downside is that if a major issue comes out, it could spread across the entire fleet. Thats exatly what happened: They leased brand new planes and then a serious engine issue occured that forced grounding of many new planes for 1+years....while they are still paying the leases on those planes.
3. The bigger competitors are really just credit card companies that fly planes as a hobby on the side. That gave them the financial ability to siphon off customers by introducing a barebones tier. Spirit customers aren't really the type to play the credit card points game so they couldn't copy what the big guys do and despite travel increasing after COVID, people decided to pay a bit more to not have the barebones experience.
4. Spirit was in the process of revamping their entire offering to better compete. More simple tiers with things baked in and less extreme customization that they used to offer. Makes sense: 9$ fares were gone for good so maybe they could compete as a solid basic tier. They did not move fast enough to turn things around. Its a bummer because it looks like they had an interesting plan after the second bankruptcy but we never got to see it fully play out.
5. The final nail in the coffin: The Iran war took out any remaining breathing room. They were just coming out of their second bankruptcy. I personally feel they had a viable plan to try and turn things around but the Trump admin just let them die without really trying. From my understanding(and I could be wrong) Trump pulled some nonsense where he wanted like 90% of the airline for peanuts, they would support ICE transport operations etc. The owners of the debt obviously said no and that was the end of that. Great job losing 17k employees their livelihood. Since this is a tenure based industry, many end up starting from scratch at another airline. A emergency merger or something would have had better consequences for the employees. Will be interesting to see how this affects the midterms since Spirit was based in Dania Beach, FL (Miami area)
The idea behind a Frontier + Spirit merger makes sense. The airlines combining together can cover a greater network, Spirit was getting rid of its poor leases through bankruptcy and maybe they could have been a major airline in the low end. The argument against is that Frontier is not doing too well either so should you really combine bad + bad?