GovA doesn't know why you exited and GovB doesn't know why you entered.
This is true for individual migrants. In case of large migration waves (e.g., East Germany to West Germany), the reasons are usually obvious.
I'm not necessarily sure we can attribute the improvements of boycotted governments to mass exodus / brain drain. Many people left China when Mao Zedong took control. But after Deng Xiaoping took control and improved China... I'm not sure if it was because so many people left or because so many of the surrounding countries were prospering while China was suffering. Now that so many of the brains have returned to China... I can't help but wonder how much this increases China's chances of further improvements.
But even if you're correct... my point still...
Historically, the evolution of government systems was mainly driven by violence, with invasions and revolutions being the principal agents of selection process. The rules of the game were predetermined by our environment - land was a limited resource, for which our ancestors had to compete, if only to ensure the survival of their descendants.
The 20th century introduced a game changer. As agricultural productivity in developed countries rose by orders of magnitude and natural population growth practically came to a halt, possessing a large territory stopped being a necessity. Countries with little arable land, ultra-high population density and no natural resources can now not only feed their population, but also achieve top living standards. These changes may open a fundamentally different route for societal evolution – one that would not be based on violence or compulsion.
A small thought experiment - imagine what would happen if central governments cede most powers to smaller territorial units:
Unfortunately, there are serious obstacles to the successful implementation of this idea:
Do you think these problems are solvable?