> As always, you have to balance the risk of a crazy trap-setting knifeman attack with the (opportunity) cost of building additional escape routes.

A "crazy trap-setting knifeman attack" is not the only scenario that results in a blocked exit trapping people inside the building in a life-threatening situation. Think about it for a second. Imagine, for instance, that a fire breaks out for any reason. Is this unthinkable? And how do you factor in emergency response personnel trying to go up the same stairs that are the only way out of people trapped in the building?

We should not forget past lessons for no reason.