Honestly IMO I think Russia and the BRICS nations could be a step towards the direction where these immigrant populations are more of a boon to their original countries than not. Russia and China in particular are two countries that could be and are doing a hell of a lot to not only export their economic/industrial model (and also collaborate academically, etc) abroad, but also bring people from around the world to see it in action, to see the culture, etc, and develop ties with them firsthand.

People are not just "lost" once they leave, and that this has often been the case is absolutely intentional by the west (destabilization, imperial plunder, etc) making the migrants' home countries destitute and dangerous in comparison, etc. And the knowledge of development and connections between people can absolutely be exercised for good and the benefit of both nations involved, particularly when they can return to their home countries with the assistance and support in recreating or building upon what they saw abroad (something the west has always interfered in on their part).

Both countries are working extensively with the global south, China with its countless BRI/etc projects and Russia as well in its Soviet history as well as presently with security assistance, debt relief/etc, and even helping build a nuclear power plant in Burkina Faso as one highlight that comes to mind (I'm sure there are others, just haven't looked into it). There's no reason that all this training/experience/etc could not be increased with talent coming to these countries to study, or with large student/worker exchanges to really develop a culture of solidarity and multipolarity. And if people wind up staying- particularly those from much less developed or unstable countries- there's still no reason they cannot be to the benefit of both countries in facilitating increased trade and cooperation, or even returning with Russian/Chinese/BRICS/etc support in due time to further develop their home countries in the end.

IMO while you're not entirely incorrect, you're thinking of things n the zero-sum model the west has imposed upon the world- there are other opportunities available, and the Soviet Union's history of doing similar, or China's presently (both without the mass immigration bits, though some immigration did and does occur) are examples of it. It's a more complex and in the short-to-medium-term, more expensive process than the western way of doing things, but I think the end result will be far better for everyone involved.