xyz_pilot,
If you look back in the months prior to 9/11, you'd see traffic was already falling off and the US economy was already slowing down significantly. (I know you've already pointed some of this out and no doubt of course 9/11 was a significant catalyst).
The giddy days of the Tech Boom were already vanishing and companies across the US were already slashing business class travel. The PAX load that fateful day of the airplanes that were used told something of the slowdown already happening.
There were signs prior to 9/11 that carriers like JetBlue were the way to go, low cost, no frills point to point. UAL and AA were/are business class airlines. When times are good they can hide their high costs by booming revenues, but like any business when they hit a downturn, that's when you can see how healthy they really are.
A recent history of Airline companies has shown some disatrous results after airlines have merged/amalgamated, PanAm and National being a good case study. People outside this industry don't realise how different employees' cultures are between different airlines and in the long run can prove very troublesome when trying to mesh them together, it's certainly not the main reason why airlines fail, but it doesn't help.