Most African cultures have not completed the demographic transition. Until that happens somehow, those cultures will exceed the carrying capacity of any environ

Most African cultures have not completed the demographic transition. Until that happens somehow, those cultures will exceed the carrying capacity of any environment, and war over inadequate resources. With unlimited immigration, the billion in Africa would continue moving until America became equally inhospitable. If anything, our population is already too high, verging on disposable "life is cheap" levels. We haven't learned to make a civilization that values individuals and treats them decently, except when they are scarce enough that resources are going underused (making it clear to everyone that we benefit from their presence).