... What if the terrorists had been successful in their attack on the economic, political and military centres?
Be useful to review exactly what the targets were.
1. White House
2. Capitol building
3. Pentagon building
4. World Trade Center buildings. One aircraft per each of the two tallest towers.
There were somme others on lists compiled by the organizers, but these four or five buildings are at the core of prosecution. The defense argues the Capitol was struck from the list & should not be included in the conspiracy charges. I think most people consider that a quibble over details.
Had these hijacked aircraft hit each of these buildings the two with potential for the highest death toll are the Pentagon & WTC. The former is a study and well complemented building but the estimates I've seen top out at 300-400 dead & twice that maimed. The WTC is a different matter. Off the cuff estimates use the example of the 1990s truck bomb attack on the WTC, the 1940s aircraft strike on the Empire State Building, and the OKC truck bomb on the Federal office building. None of these are very good comparisons to a modern fuel laden airliner. Since we don't have any close examples the question is wide open to argument. The peculiar construction of the WTC, a exoskeleton cross tied to a core stem has its own engineering peculiarities. As with the Empire State building the impact of a airliner is not believed by architects/engineers to be enough to collapse the building. Two striking the same tower could. The consideration of fire is a bit more clear. The principle fire safety feature was the steel columns were clad in a insulating are resistant sheath of concrete. That is estimated to be able to withstand the temperatures of burning aircraft fuel and office furniture. So, how many people are killed by the impact and the fires in the upper tower/s with no collapse. If the hijack pilots do not follow the plan and two aircraft strike a single tower to collapse it, the all bets are off. A collapsing tower of the WTC during working hours on a week day has the potential to kill thousands. Perhaps over 4,000. Maybe even 5,000.