The basic tally is that (2) skyscrapers total, in the history of skyscrapers, have been brought down by airplanes crashing into them. Any less than that, and the stairwells would have stayed fireproof long enough for people to get down then. The people above where the planes struck didn’t get out, the people below did, ergo basement bunkers would not haveh helped.
Helicopters couldn’t rescue people off the roofs because the heat was too intense, and the fires were, anyway, burning too far down for water drops (as Poser said).
Inflatable bridges are ineffective, for the same reason as my first argument—the stairwells would have held had the fires not been caused by a high-speed airplane impact.
Emergency parachutes, same deal.
You have to remember that most fires in skyscrapers are relatively contained, and don’t include impact events that break up fireproofing and stairwell walls.
Security and safety are tradeoffs—as the adage goes, we can’t build airplanes out of the same stuff that we build the blackboxes in the airplanes, becuase the roads aren’t wide enough.