Back in the days when people seriously regarded Trump as an opponent of globalism and open borders, rather than being just another neo-conman stooge, Salon writer Anis Shivani wrote a piece, “Everyone’s Wrong on Immigration: Open Borders are the Only Way to Defeat Trump and Build a Better World.” : http://www.salon.com/2017/03/15/everyones-wrong-on-immigration-open-borders-are-the-only-way-to-defeat-trump-and-build-a-better-world/. He quotes studies showing that world economic output would double if people could freely move wherever they wanted, which means, to the West from the Third World in their billions. Nations are not like homes he says, but are abstractions, only as good as the freedom within it.
This sort of argument is advanced purely for the West to take migrants. India, China and Japan are exempt from taking in the tens of millions of poor Africans, or the billions of them in the future. To suppose that this would increase economic output is just economic theology, because it is more likely to lead to the collapse of the West rather than any economic revival. Let India and China take in proportionately more migrants than the West, and then maybe we should think about it.