Your point is correct: if they knew the accident was going to happen today, they might have changed their behavior.

Unfortunately, nobody who knew it would happen today warned anybody.

Yes, but there’s a difference between what people will pay in advance to prevent one of many low probability catastrophic failures and what they’ll think was worthwhile for someone else to have paid to prevent the one which actually happened.

Those calculations are really hard: say they had built a tunnel, what are the odds of the same number of people dying in a fire after someone crashes into another car? Would we have needed to spend any money at all if more people had used railroad alternatives to driving and such an expensive bridge or tunnel was not justified on traffic grounds?