They keep costs down by only flying select routes, from cheaper airports if possible, one type of plane so less training needed for pilots,mechanics, spare parts for only one type of plan. No company retirement when 2 friends went to work for them so most pilots were retired military with a military pension to supplemrnt SW pay. If flight isn't flown- crew doesn't get paid.(again last i heard from friends)
I have flown them but if every airline tried to copy SW there would be far fewer cities served. SW only flies when enough demand between city pairs and can't easily adapt with smaller or larger aircraft or gather people at a hub , sort, and send on to final destination.
Airlines were originally regulated to ensure flight service to most localities.
Don't know what will happen now with most airlines in financial staits and oil going up.