The use of airplanes in terrorist attacks wasn't a novel idea.
The Metro Toronto conducted a disaster preparedness exercise for a mass casualty event which included a plane crashing into a chlorine facility. That was the late 80s or early 90s.
There were foiled plots to crash a plane into the Eifel tower and the CIA buildings in the 1990s. There were quite a few airplane bombings in the 80s and 90s, including the Air India bombing in 1985.
(Getting old is weird af, lol. It's strange watching context dissolve from the collective memory).