> I don't think it follows that preventing that brain drain would have lead to appreciably better outcomes for those countries.

Well sure, it depends what the counterfactual is. If those countries just physically prevented the people from leaving, and nothing more, I wouldn't expect that countries' outcomes to improve. But what the countries suffering from brain drain presumably want is for there to be attractive opportunities for those skilled workers in their own country.

Gifted architects and builders are presumably born every year in Silicon Valley, but we are far too rich, developed, and democratic to want new buildings.

Other countries are free not to want the things that Silicon Valley talents generate. More for us!