Urban culture is unsustainable if hell breaks loose, just saying.
What does that mean, though, 'if hell breaks loose'? Zombie apocalypse? Nuclear winter?
Yeah, mainly. People living in urban environments can be trained to be self reliant. The Walking Dead TV show sort of plays a role.
I've always assumed metropolitan areas would be the first to go in such events. In a full nuclear exchange, assuming they were one of the few metro areas not directly targeted, they would tear each other apart through looting, starvation and crime in general. Others would leave the area but be somewhat helpless as most of them would probably not have any resources outside of the city.
For the sake of debate, in a zombie scenario, urban areas would also be decimated and the first to go. Too many people clustered together with no real time to react (depending on type of zombies!) and with the bulk having no general survival skills.
Cities/metro areas are only possible because of our modern support systems. If any/all of them break down, the cities fall apart.