All the low cost carriers are in trouble.  Southwest is barely breaking even, and is rolling out more premium features to try to stop the bleeding.  

There is a market niche to providing ultra low fares, but they also get absolutely zero premium passengers.  All of their customers are very sensitive to inflation, and price increases, because it will be years before their wages catch up.

Airlines that have business travelers or contracts with major corporations are able to raise fares to match increases in fuel and labor costs, because businesses that need to fly have already raised their prices:  they don’t need to wait a year and beg for a wage increase, like 100% of spirit’s customers do.