The idea that the West would revert back to tribal Nomads isn't accurate I don't think. Just because we don't have 20th Century tech doesn't mean we don't have other tech that would allow people to survive and thrive. The folks that settled the plains really didn't have any tech the romans didn't have, save for steam and things that go "bang". They would still know how to channel water, build decent buildings, etc. Ranching is still largely unchanged. If the Persians and Moors can build Empires in a desert we can sure get a couple on fertile prairie soil.