Its even worse than you think.
Southwest's fare is very rarely the lowest in a mature market, but they are NEVER the highest either. They developed a reputation for "fair" pricing, so people don't even shop around. That is what you call brand loyalty, and it wasn't done with fancy services, glitzy marketing etc. Just fair pricing, not consistantly screwing your best customers.
Yeild management was like crack for airline managment. They got addicted to it and used it to cover their short comings. Then when they load factors would go up they would raise prices, increase their yeild and kill their own market. When Southwest's load factors go up, THEY ADD ANOTHER FLIGHT, instead of jacking prices, which increases the utilization of their equiptment, increases their market presence (makes em more convienient) and makes the MORE immune in minor fluctuations in traffic. (If you are flying 1000 people between a city pair and 50 people stop because of a downturn in teh economy, its not as devestating as if you fly 100 people and 50 stop flying)
Yeild management made for some spectacular quarterly reports at the expense of long term traffic growth.
BOHICA