In such a world, why wouldn't you see 1. foreign R&D companies, 2. indexed into a thriving foreign equities market, 3. gathering the interest of domestic investors who want to diversify beyond domestic investments, by 4. moving their money and/or investing in domestic proxy investments?

I say this as a Canadian whose managed mutual-fund holdings are apparently largely composed of foreign (mostly American) proxy equities — and who has met many Canadian-based VCs who don't do much investment into Canadian companies. If not for talent immigration, the American investment landscape would probably look similar!

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.