They're no longer low cost because of service erosion at the main carriers (squeezing more seats in, getting rid of baggage allowances, etc.). As a result of that erosion and southwest's holding the line, they are now one of the higher quality carriers (and some would say one of the best!). An interesting course of events to get us here...

Spirit & Frontier (and Ryanair in Europe among others) are actually classed as ULCC's (ultra low cost), where everything is nickel and dimed.

At the higher end, it certainly isn't necessarily the best any more, but I do think that's mostly because Southwest has kept to its two free checked bags, whereas the major carriers now make significant revenues from checked bag fees.

It took one trip for Spirit to go onto my personal No Fly list.

While Southwest remains my favorite airline.

I took my first and last Spirit flight ~7 years ago to fly cross-country. It honestly felt at some points like the plane was going to fall out of the sky.

And Frontier. They're the first one that comes to mind when I think "cheap".