There are many reasons societies crumble and reorganize. Rome became over extended militarily and corrupt from within exactly like America. Most Romans were really shocked to find barbarian tribes raping and pillaging them. The corruption in Roman society led to their slow demise over several hundred years until they were replaced by other power centers. However, many of the indigenous population carried on living even though their empire died.
I suppose the moral of the story is societies are born, they thrive, then decay from corruption and loss of virtuous behavior. They will be replaced by other power centers. The trick is to stay alive during the transition from one gang to the next. I want to grow as much food as I can and have stable sources of drinking water. I am glad I live outside the city where most will not be prepared and therefore desperate. The thirty miles between me and them is a barrier that the majority of city dweller will not be able to cross without gas and a car. Still, I think it's a good idea to grow more than I need for the few who are able to walk out alive.