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.
You make a good point. We, the allies, bombed Berlin to rubble and then two Russian armies set about redecorating the place while the Third Reich had its last gasp and old men and children set about to make the remodeling job as messy as possible. Then it was split in two. But Berlin remained a great city.
It is late, early, and I can't come up with any city of any size that was abandoned in the last few hundred years.
I go down a list of cities with major issues and disastrous conditions and I can't think of one where people just gave up.
Berlin, Hamburg, Dresden, Hiroshima, Nagasaki, Tokyo in WW2. Sarajevo during the Balkans conflict wasn't abandoned even as it was blasted, tortured, and converted to an open air shooting gallery. Baghdad, Fallujah in more recent times.
Cities are remarkably resilient. City dwellers are quick to adapt and quite tenacious.
I suppose that there is some degree of damage that would force a city to be abandoned. Soviet war planners had, according to one account, a plan that included 24 nuclear warheads, in sizes up to twenty megatons, going off over Washington DC. Hard to imagine rebuilding but I'm not entirely sure about it.