I don't think anybody discounts it or hand waves brain drain away. My concern is that these notions might inform (well meaning) ideas of what immigration should look like - it feels overly paternalistic and minimizes the immigrant's desires and agency. Immigration is already an incredibly difficult and uphill task that's near impossible for the vast majority of people even with the means and desire to do so. However difficult you think it is, multiply by a factor of 10. So now should the process be even harder because of concerns about brain drain? For example, should American concerns about brain drain from the underdeveloped countries inform if people should be allowed to immigrate from there? Should immigration be open only to people from rich countries?