I doubt we're reaching the apex of magacities and I don't think we'll see the sudden collapse of one either. Unless personal transportation is so improved as to make travel from rural areas to the city cheap, fast, and effortless, more people will move to cities. Baghdad is a good example of how resilient large cities can be. Since the early 90's the critical infrastructure (water treatment, electricity, etc) has been in shambles and still is today. Yet even with sanctions, invasion, ethnic cleansing, and horrific daily violence, millions of people still live there. How much worse could it get than that? Nothing even remotely close to that will ever happen in an american city, so I think we'll be fine. As for the popular survivalist scenario of a mass exocus of desperate citydwellers ravaging the countryside for resources I say, keep dreaming.