It's really simple.
How many times - as part of a single, co-ordinated action - have airliners been used as flying bombs?
Answer: once.
How many times have aircraft suffered from pilot incapcitation, rapid decompression, loss of flight deck windows, birdstrikes/hail smashing cockpit windows?
Answer: loads.
The chances of a September 11 type attack happening again are miniscule. It was a 'spectacular' that would not have the same impact if repeated.
However, by causing the West's airlines to run around like headless chickens spending millions unnecessarily - and by deterring the pax from flying - the bad guys have won.
Don't let them.