This scenario is a fun thought experiment in science fiction stories, but this is so astronomically unlikely, you can plan around it. It is fairly stupid to make plans for this kind of scenario, because you will probably die before it gets that bad. You live in a city? You will be dead long before "things got really bad". The worst case scenario worth planning for is a totalitarian government. Martial law that gets extended indefinitely, and a general that becomes "President". There would be famine generally, and loads of people will starve to death. But independent roving warlord raiding bandit tribes ("feral" authorities) like in Mad Max? Not happening.