"Even leaving terrorists out of it, there is always the possibility of a civil airline crashing into a building, and the public had always been told that buildings like that were so tough they could withstand anything like that."

Sure. Have you seen the photos of the B-25 bomber that crashed into the Empire State building?

Empire State Building Crash

Architects thought they had actually improved building design to survive larger planes. They just didn't anticipate all that fuel burning and melting a handfull of beams necessary to keeping the builds up.