A thought on the pax/freight asusmptions. It seems that the accepted wisdom is that UPS/Fedex have it made forever and a day. Very likely. But, consider this scenario. There are other smart businessmen in the world besides Fred Smith and the UPS trucking guys. I suspect that someday a third or fourth company will make a real go at the cargo side of flying. DHL is huge everywhere but North America (or so I'm told) and made/is making an attempt to compete here. UPS/Fedex have been riding a comparative gravy train. A Soviet Union/USA kind of thing, watching each other, tit for tat (you buy Kinko's, I'll buy Mail Boxes Etc. looks like Fedex got the best of that deal). If and when a third or fourth competitor enters the market with a junior workforce, new planes, no DB pension (you know like so many of the LCCs, one of which I work for) it may cause a seismic shift in the cargo industry. As in, contracts up for renegotiation, etc.
A positive about SWA (besides being a place that I can see myself still flying in 20+ years and being happy about it), we've survived the cutthroat pax competition so far. The fight is never over and we've got our eye on costs. But competing against and surviving amongst 20+ carriers, all trying to make money, undercut the other guy, etc. That means we are lean and a survivor. Not saying that Fedex or UPS are not lean, just that it really hasn't been proven in the Darwinian contest that is the pax airlines. As long as Fedex and UPS have similar costs and stay more reliable/cheaper than US postal, they have a good thing.
The big problem with my pet theory is that cargo entails a HUGE up front cost to get the storefronts, trucks, drivers, space at 200+ airports. Whereas pax carriers can enter the market with 3 planes and 2 airports. Can't do that in cargo so it will take a Richard Branson, the 'Easy' guy, or a serious effort by DHL to enter the market. But, there is money to be made and that means people are looking at ways to make it.
Short version. Cargo has a good thing with the Fedex/UPS duality that may not last forever. SWA has survived and thrived in one of the most capital killing industries ever invented by man (knock on wood, so far).