alt Hacker NewsWhen people have no hope of not making it as a permanent resident or citizen, their incentives to perform well are not as high. Also immigration is a global market, you compete with other countries that might offer better conditions so you lose on the best workforce.
I don't know the overall ratio, but my experience working with many immigrant workers is that they had no real intention of staying and instead are just arbitraging cost of living between [rich country] and [poor country] for their family back home. Emphasis on home.