No idea what the response would have been at that point, but if a truck bomb had been detonated at, say, Memphis International (back in 2001, an important regional airport and of course a major air freight hub) and killed 1,000+ (not hard if the terrorists attacked at the right hour on the right day), I'm just guessing that airport would be shut down for a good while (if they specifically focused on the freight operations there, like the FedEx Flight 705 incident, the result is even worse). IOTL, they already had won with their impact on air travel, anything else, like attacks at major air hubs like Los Angeles, Frankfurt, etc., is just "running up the score". It's very fortunate al-Qaeda decided to go for pure shock (i.e. destroying major landmarks) and not for pure casualties. Especially when Khalid Sheikh Mohammed had a good clue on how to do a terrorist attack against the US.