Someone somewhere, perhaps, signed off on NOT installing a correct, suitable worse case, thoroughly tested - often- backup system. Heads might roll, but don't hold your breath. They can not pin 'pilot error' on this one.
Someone somewhere did not do a thorough threat/risk assessment of 'what happens if...../'
Pilots, controllers, firemen, etc are taught to think "What happens if..." They are safety personnel, and need to always have a backup plan or even two.
But if you work in an office or cubicle, you get hammered for always asking "Yeah, but what if...." Then you're crapping on someone's plan and budget, and delaying the project. Those people do not think like we do, never will.