i honestly believe that the beginning of the downfall began immediately after 9/11/2001. That date ruined lives and has continued to ruin life in general but specifically for travelers up through to now and will likely keep doing so. (of course even that was just one dominoe among so many going back that each generation can only count from when theirs began, but the previous generations just sadly laugh as they remind the young how it keeps going back).
Anyway, as far as the airline business in the U.S. goes that is the marker of decline. After the TSA began and dug its claws into the passengers and the airlines returned the favor to the customers by punishing them for even thinking of using their service, you get what we have here today. The sad part is no other travel form has stepped up its game to favor the individual. Trains had the chance to make a diffference like they do in every leading country (Japan, here’s looking at you) but they too didn’t bother. At this point i think it would be easy for a fucking Zepplin service to startup and treat customers right with a good price and they’d come out ahead.