Spirit might seem to be on an upward trajectory, but it's an illusion.
All their expansion was based on being allowed to continue fooling this nation full of stupid people looking on the internet for the cheapest airfair.
Now that the airlines are being forced to reveal/include all of their various up-charges in their internet fares it's like Spirit has had their feet swept out from under them. Yeah, they still appear to be going along great but in reality the air has just been let out of their balloon.
Look at the financials. You couldn't be more wrong. Like Ryanair in Europe, Spirit is the future of the industry like it or not.