You are 180 degrees off. This is the new era of decentralized living. The economies of scale that prompted the growth of the older cities has been overtaken by the increases of efficiency of distributed services and the costs of maintaining an urban core. There is nothing efficient about crowding 20,000,000 people into a small, difficult-to-maintain space.
If decentralized living is more efficient then we will see a trend towards it. I'm not sure it can be as efficient as a downtown core. For example I was reading about the Chinese putting in high speed internet. It doesnt' take that much to thread wires underground in already designated tunnels, t hen hook up 40 connections inside a building. Its a lot harder digging up 150 feet to install per one residence in exurbia. Then the servers and e-infrastructure is the same.
One thing that could really be game changing is if a lot of the good jobs can actually be done from far away, like in India almost as easily as down the street.
Can we really generalize like that?
Your vision depends a lot on low energy costs. It may also not be practical where free land is limited.
You can spread people out to Vermont or Montana, but a city like New York or Los Angeles is going to need a reason for being if it's to go on and prosper. Will it find one in complete decentralization?