i thought it would be easier for the airlines to add more focus cities/hub schedules...many of the airports that renovated/built during the 70/80/90s only see a small fraction/regional to these overcrowded cities...here are 2 examples that I can think of off the top of my head. I also think that the US/Eu agreement is helping this factor...for US Airlines to service EU, mainly flights are lead into connecting hubs of EU major airlines...and the same with EU carriers to the US...both cannot be blamed because airlines are trying to reach their travelers to the destinations easiest by going directly to their hub.
Ex- BAW services JFK, EWR, IAH, DFW without many other destinations while for example DAL is basically forced to Gatwick and Frankfurt...drawing airlines into these places
Pittsburgh International- USAIRs prime hub late 80s/early 90s with many overseas and domestic flights daily brought a complete new airport to the region...now there are hardly any direct flights besides to overcrowded hubs/focus cities.
American Airlines seems to be paving the way for fixing this problem---Lambert (KSTL) became a reliever for American's existing hubs at
Chicago O'Hare and
Dallas/Fort Worth. American transferred many mainline TWA routes to
American Connection, a group of affiliated regional carriers.
Not sure here though to be honest