Here is what I think would have happened:
Now, prior to 9/11 there were a series of sweeping media-attacks on airlines, such as a somewhat slanderous special feature on MSNBC and a series of anti-airline scenes in the popular 2000 comedy "Meet The Parents." The airlines would probably have had to come up with improved luggage handling etc.
TWA had already been purchased by AA
and I strongly doubt that AA
would have allowed them to continue as a seperate entity in a non9/11 world.
United probably would have suffered severe problems as the economy was in a decline and United had gradually been developing some performance difficulties.
So basically, we would probably see the same airlines we have today, but they would be larger, healthier, and perhaps offer marginally better service. Also, wait times at the airports would be substantially less. We can hope that 10 years from now we will actually see such a scenario.