If the designers and the engineers and the planners didn't anticipate that planes were going to get a lot bigger within the lifetime of the buildings they were just irresponsible fools.

The Towers should not have fallen, not as quickly as that anyway. What was to be expected was that the structure would be badly damaged, and might even need to be demolished. The collapse came because of design flaws, in a design which failed to take account of the possibility of a large aeroplane crashing into it with a full fuel load.

Planes fly over New York. Planes crash sometimes, normally by accident, but they still crash. And when they crash they have to hit something, and in New York that is probably going to be a skyscraper. So when you build a skyscraper, that is one of the things you   design it to withstand.

And they had in fact said that the Two Towers was designed so as to withstand such a collision. You can't blame people for believing them.