First of all, elevators never plummet down their shafts. For the past century, elevators have had a backup break that automatically engages when an elevator starts to fall. If all the cables snapped (highly unlikely), the elevator would only fall a few feet before the safety breaks would activate. The safety breaks are mechanical so that they work even if power is out or no one is around. Secondly, even if a terrorist managed to saw off the huge metal breaks and cut all the cables, the plummeting elevator would land on a cushion of air at the bottom. The elevator shaft traps air much like a giant air bag, which would soften the blow. But the essence of the original question is still interesting.