The trend has already begun without any major cataclysm, only as a result of the industrialization process we began 350 years ago.
More people move now than ever in history, there are more economic refugees and wealthy expats, and a whole degree in the middle, than ever before.
The USA which has during the 20th century represented a New World, a new way of doing things, has became distinct in the World as the only country where people of all social strata move from one end of the continent to the other for positive economic reasons (ambition) as well as necessity. Teenagers chose their college quiet irrespective to geography and adults don’t hesitate too much to change state, climate and social circle to work a better job.
In the 21st century Dubai has grown to an International City not because of its native population, or only because of its oil, but thanks to the economic migrants and the expat jet set that patronized it.
The migration to the big cities Worldwide is ever increasing and problematic, posing questions that can only be answered with further mobility.