If you're going to plan for all of the one-in-a-million events, you're not going to be able to get anything done. It's more reasonable to round small probabilities to zero and deal with them on an ad-hoc basis.
This is not a one-in-a-million event, this is not something that no one could've predicted, many people predicted it and we blithely ignored them. This is the direct consequence of our consistent policies and behavior. This is like standing 99 feet from a cliff and saying "well the next 100 feet in front of me is 99% solid ground and 1% air, sounds safe to me!" and then walking right off of the cliff.