I don't really buy that last one: successful LCCs (Ryanair, AirAsia, etc) know their niche and their audience, and don't even try to compete outside it. Luxury travel has fatter margins but also demands way more overhead in everything (cabin crew, catering, cleaning, etc), so it's not compatible with the rest of the LCC business model, which relies on fast turnaround times and minimal services.
I think luxury is the wrong word. I have a hierarchy of US airlines in my head that guides my domestic airfare purchase decisions and Spirit is at the bottom. American and United are near the bottom too. I never fly first class or business class, so I'm always looking for the least miserable coach ticket. For me, that's Delta, JetBlue and Alaska. I imagine a lot of fliers think the same way and though their own rankings may vary, Spirit is at the top of nobody's list.
And what's the difference is cost anyway? Maybe $40.
@eskibars was specifically talking about luxury. And yes, there are people who will fork out $10000 or $50000 for a 10-hour flight where they get their own little multi-room apartment complete with a shower and free flow champagne.
