Brain drain happens when the quality of life in a particular country dips below some standard. It's relative to other countries only up to a point. I strongly doubt it is possible to increase quality of life (albeit for a minority) in one country so high to outweigh the utility of staying put in other countries. You certainly can't do it in a democracy: who would vote to increase the quality of life specifically for a small group of immigrants? I'm pro immigration btw but people are not going to vote against their own interests to such an extent.