A lot of poor, homeless and disabled people would move to countries with welfare for everybody, like Norway, until they'd get so overpopulated with freeloaders that they have to change their policy. Other popular countries like America would get flooded too, cities would probably get a lot bigger and there'd be a lot more of them. They'd flood to the degree that the downsides of overpopulation would equally offset the original benefits of living there, then equilibrium would set in.