The company is not forced to sell immediately to whoever offers it money, they can sell themselves off for parts.

I heavily doubt PE firms are interested here as there is no potential for growth or a multiple. Spirit's assets are mainly their fleet, there are like 4 maybe 5 people who could buy, of these 2-3 are facing similar financial crises.

In the US I think nobody except United can afford to make a move, more likely some Asian airlines will move; many have grown and have route demand they can't service due to lack of aircraft. If you fly to Asia often you'll note that much of the time Asian airlines have to operate an aircraft from a US airline.

Spirit's assets are almost entirely their slots at ORD, EWR, LAX and LGA. They don't many of their planes.