Yet strangely, all four metropolitan areas have experienced tremendous housing price appreciation. Detroit, Cleveland, and Pittsburgh are in decline but with cheap housing.
The young people I know leaving Boston do so because they can't afford a home in which to raise a family. The job market and quality of life are still excellent here, as in the other metro areas, but supply and demand for housing are out of whack so the people who have the most to gain (young people with no current toehold in the housing market) have been moving elsewhere.
But don't discount political motivation on the part of substantial numbers. For example, in my state (Wisconsin) very large numbers of retirees depart the state for weather reasons, but also because of the crushing tax burden.