Great post, Shanta, thanks.
There are persuasive instances of brain gain over the long run, as the exemple of research capacity building in economics in Latin America over the last decades seems to show. However, I don't think one should systematically expect brain gain, as suggested by work by Frédéric Docquier and others. Another caveat to your story is that labor mobility within countries is not always a cause for celebration, and the pace of rural exodus toward cities in the developing world while there is a lack of provision of basic services in the cities raises many concerns: actual labor mobility does not always imply moves from low productivity to high productivity jobs, and neither does migration.
In fact, you might make almost similar arguments for trade in goods and services (domestic vs. international), even though resistance to migration is probably higher and deeper than resistance to trade, notably because the mobility of persons is of a different nature. However, there is the same analytical slip there: benefits from trade are (presumably) more easily understood and accepted for domestic trade than for international trade. I was always intrigued by the fact that trade theory only addresses international trade (thus taking politics as given). I guess that beyond the factor mobility argument (which in fact may also applies domestically, for example when there are patent infrastructure gap like in many African countries show), much has to do with the financial and social “sustainability” of domestic mechanisms needed to mitigate the costs of transformation, as you suggest in your post. We might need to study not only the resistance to migration but also, more generally, the nature and multifaceted dimension (fiscal, economic, cultural, political, administrative, logistical, technical…) of the resistance to things foreign, i.e. the separation between “domestic” and “international⍊ and to what extent, or whether, this resistance is endogenous or should be taken as exogenous. Which echoes the work done on why there are countries and whether it is efficient…, but I don’t think this work has gone far enough, and it should involve many disciplines. Yet, it also implies discussing the effectiveness of the "international market mechanism": is the international reallocation of resources, including labor mobility, related to any market failure or to an efficient functioning of markets? Is there an optimal human resource allocation across countries, and is there room for an international “politique d’aménagement du territoire”, to echo one of the French public policy concerns across the French territory, as regions may fill up while others become empty?