I used to think this until I realized that the theoretical point isn't to prevent great loss of life, it's to prevent the usage of airplanes as a
weapon. Malls, subways, concerts, sports games were always target-rich...targets, but a plane can be used to attack infrastructure and militarily significant targets (e.g. the Pentagon) and to kill far more people with lighter weapons than you could otherwise.
Of course, this could never happen. It's been said to death, but the conventional wisdom before 9/11 was to allow plane hijackers to run their course and assume that all they wanted was cash, to just sit tight and wait for them to set down somewhere. Not even after, but mid 9/11 people realized that old paradigm was out and the passengers of the fourth plane the cockpit door, preventing the usage of the plane as a weapon.
Post 9/11, nobody could hijack a plane with box-cutters anymore - enough people would rather be stabbed than see their plane be used as a weapon that it's simply insufficient, especially since if they crash the plane you're probably dead anyway.