It hasn't made any positive difference leaving the present brains in Russia - look where we're at. Those brains that ideologically align with the liberal West are presently trapped inside of a Russia they are likely to fundamentally disagree with, we should bring them over to our side if we can.

What will be left? The same power structure and malevolent culture that already exists, minus some of the brains.

I believe the last few rounds of brain drain out of Russia have helped to leave their scientific, aerospace and military programs in the relatively dire condition they are at present vs the Soviet past (emphasis on helped, there are other factors such as eg Ukraine no longer being part of the USSR, with Ukraine being an important contributor to various Soviet accomplishments). It has net weakened Russia considerably in my opinion, that's a good thing, until or unless their culture changes.