Again, SWA never nickle and dimed the customers to death. There's only so much people will take.
Not sure how often you buy tickets, but I buy (and recommend) often, and I have to say one of the most expensive airlines out there for domestic travel is SWA. Spirit is cheap, only because they've taken everything out of the base fare. SWA is expensive because everything is included, 2 checked bags, drinks/snacks, etc. There is NO such thing as "free bags!" You pay, everyone pays for it on SWA. It's just included in your price for the ticket. For Spirit, I've seen that even adding bag fees (28 for the checked + 30 for my carryon) the fare still comes out much lower than SWA's fare in many markets.
And a lot of people today are driven by price for airline tickets. Loyalty can easily be changed if someone else can fly them to their destination cheaper. Spirit has adapted to the changing environment and has been successful, announcing around 30 million in profit every quarter in 2011. If you can make money in the worst of times, they'll definitely be around for quite some time to come!