More than most other topics the World Bank analyzes, migration is a political rather than an economic issue. Resistance to migration can therefore only be understood by adding a psychological / sociological point of view ('fear of the unknown')to pure economics. I agree with the general arguments made in this post although they are hardly new and have been advanced repeatedly as part of the GFMD process. Just a couple of points.
1.) Any attempt to look at migration through an economic lens is inherently flawed. The benefits of migration cannot be equated with total gains for the global economy. This type of economic modelling does not take into account social costs to migrants, their families and host populations.
2.) The effects of migration on receiving countries vary. Studies in the UK by Ruhs & Silva (2012) (http://www.migrationobservatory.ox.ac.uk/sites/files/migobs/Briefing%20-%20Labour%20Market%20Effects%20of%20Immigration_0.pdf) show that the effects depend on migrants' skills levels. Moreover, benefits of migration are felt differently by different groups within the host society. They are generally largest for employers and smallest for unskilled native workers, who compete with migrants over low-paid jobs. The care sector in particular is prone to such competition. Such findings need to be taken into account when understanding resistance to migration in receiving countries.
3) The brain gain argument is complex - in the case of many developing countries higher education as a means to facilitate going abroad is part of national culture ('culture of migration'). This is problematic insofar that individuals tie their hopes and dreams to jobs abroad that might not be attainable to them, and also endangers their social relations. Furthermore, long-term gains for sending countries depend on whether their labour markets have the capacity to absorb these highly-skilled individuals. Often this requires a transformation that takes time and political will.
4) Preventing citizens from emigrating is deeply problematic and touches on human rights. However, it can be justified should the education have been paid for by the tax payer, in which case graduates can be required to reimburse the schools. Paying competitive salaries for businesses and institutions in developing countries is unrealistic. Instead, migration becomes a cost-benefit analysis that includes social and economic costs. Finally, whether remittances outweigh the costs sending countries spend on education depends entirely on the external environment. Plenty of studies have shown that remittances are no guarantee for sustainable development and studies have shown that they decrease over time as migrants become more invested in their new home countries. If invested in education they can encourage change at the household level. However, if not absorbed by the local labour market, well-educated individuals will follow their parents' example and go abroad thereby only contributing through further remittances.
All of these arguments are well-known but it is important to keep in mind that migration is always a product of a vast variety of different factors and cannot be treated as an economic equation.