It's a transformation of the U.S. economy from an agriculture economy to non-agriculture. People who are left are the elderly who are dying off. The replacement of industrial jobs will continue to expand parts of the Midwest and blur the line between the Rust Belt and the Sun Belt, regions that were defined in the early '60s by huge changes in job creation. People fled south after factories shut down in the Rust Belt. Exurbia will expand around thriving urban centers like Atlanta, Nashville, Phoenix and Denver.
Demographers point out that economic conditions are unpredictable.
One downturn could trigger a dramatic change in population in some regions. For example, if the oil industry slumps, people will leave Texas and Louisiana. We're not able to predict inflection points in human behavior.