Members of the middle class will (take) matters into their own hands by forming suburban collectives to share the costs of security --- as they do now with education – and shore up delivery of critical services. These “armored suburbs” will deploy and maintain backup generators and communications links; they will be patrolled by civilian police auxiliaries that have received corporate training and boost their own state-of-the-art emergency response systems.[emphasis mine](h/t William Lind for the quote<)
Dispersal and hardening works really well when the threat is a fairly indiscriminate, large area weapon or system of attacks. The best example of this defensive response was the dispersal of ICBM silos so that any single incoming warhead could only target and not necessarily destroy a single silo. However, unless the residents of the 'armored suburb' are either exremely wealthy or are willing and able to pay significantly higher taxes, the probability of a large number of these armored hedgehogs on the exurban fringe becoming economically feasible are minimal.
If I understand the concept correctly, these communities would be self-contained and self-sufficient for short to medium lengths of time if the multitude of networks in which they participate in (medical, water, waste disposal, electric, natural gas, petroleum, communication, economic, security) were to be cut or at least severely restricted. The self-sufficiency would be achieved by a combination of local production of the minimally needed goods and services and the maintenance of large reserves of critical goods.
Building these networks out of whole cloth are very expensive.
A superior alternative may exist in restructuring the nature of existing urban infrastructure if the primary threat are discriminating, localized attacks. Cities have multiple redundancies already built into their infrastructure and rapid reconstruction/re-routing capability. If this is combined with an increase in micro-local production of critical goods and services such as fuel-cell electrical generation, urban wind farms, small cistern storage to relieve rapid draining from storm water due to the high level of impermeable surfaces in urban environments are some of the simpler resiliency increasing activities that can be undertaken today or in the near future.
Additionally dense urban environments create a multitude of demand reducing elements. For instance New York City's residents are demand significantly less energy per capita than the typical suburbanite or average American demand load. This reduction is gained through the dense interconnected and fairly resilient transportation networks, the multi-use neighborhoods that dominate the city and the shared walls and high building density that allows for lower heating, and cooling costs.
I can see the elite of the elite further isolating themselves from the American urban environment, as that has been a trend of the past hundred and thirty years in the America. However for most people, there is a strong argument that medium to high density, mixed-use urban infrastructure provides greater resiliency from a series of attacks today and in the future at a lower total and individual cost than retrofitting existing suburbs or building new communities on the exurban fringe.