The fires heated the structural elements of the towers supporting the structure above the fire floors, having the same effect as a controlled demolition would have. Had those structural elements all not been heated similarly and simultaneously, the upper part of the structures would have toppled, collapsing at the site of the greatest weakness first, and causing the upper part of the structure to fall over rather than remain more or less vertical and pancake the floors below it.
In that sense, the effect was similar to a planned demolition (of any building in a developed area) which would not have toppled the building, but caused it to fall vertically, more or less on site.
And of course the building wasn’t designed to take a hit like that. The insulation on the steel supports was blown off, the central core damaged and a sudden hot fire from the jet’s fuel.
Who could build with that possibility in mind?