Entire regional economies depend on the influx of talent. Without immigration, even the home to the world's largest technology companies would be suffering.
A captive labor pool (like working mothers who are unlikely to move their children) drives down wages. And as labor becomes more captive over time, the divide between rich and poor grows wider.
Return migration—native sons and daughters coming home after cutting their teeth in a Big City—is responsible for a burgeoning world-class restaurant culture in previously forgotten or ignored locations.
Chicago, where many academics and icons have moved to try out new ideas, makes the case that migrants are more important than density to the greatness of a city.