Since when has ANYTHING been locked into the fares? And FWIW, my wife has been shopping fares for a long time and has not found SWA to be any cheaper than anyone else, typically. At least not for what she was looking for.
Interesting facts (perhaps I am better-acquainted with SWA because they are my hometown airline).
Southwest never claims to be the cheapest and, in fact, they usually are not. Other than when they are opening up a new city, they generally price their inventory at a rate which is sustainable.
You'll notice that they don't have dozens of fare buckets, ranging from "give-away" to "holy cr*p!"- instead, they have very simple fares, long lead advance, advance, and walk-up.
But, where they really excel is in several key areas:
1. While they may not be the cheapest, they are never ridiculously high, even when they have little or no competition on a given city pair. If you want to see this in action, compare fares between SWA and another carrier on a pair they both serve, then check the fare for the traditional carrier on a trip of similar length on which SWA does not compete- you'll almost always find that, if SWA is not there, the fare is MUCh higher.
2. Fare rules- no change fee ever- if you have to change your flight, the worst you do is paying the additional fare, if it's higher, but you never ever lose the money outright. Most carriers these days have either non-refundable "use it or lose it" fares, or they charge $100.00/head.
Southwest started as a carrier catering principally to business travelers, and these sorts of policies have helped them keep them.
Also, don't forget about baggage- SWA does not charge to check bags; I forgot about that when we booked our recent vacation, and booked AA- $15.00 first bag, $25.00 second bag. Each way. Zowie.
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Not like they're perfect, but they treat passengers with a reasonable measure of respect, which (I believe) follows from the fact that they treat each other with respect. Any SWA employee can walk in to the HQ on Love Field, and walk into the President's office, w/o an appointment. Fact. And, as you walk down the hallways in the HQ, there are all sorts of displays on the wall, showing all about various employees' lives- their family, history, hobbies, whatever matters to them.
If I have to fly commercial, they do well- I can book with them secure in the knowledge that I am not getting gigged, and if my plans change, I am not throwing money in the dumper. It's reassuring.
All that said, I'd rather fly myself in most cases.