I don't think it is credible to say the UBI could replace all other welfare. Even if we were able to do an extremely generous UBI of $30,000 per year I bet there would still be individuals with either extreme needs or extreme irresponsibility who would need more than this. Given the current moral climate I don't think people would say "You've had your $30k so we're not helping any more" - instead something like the current welfare net would remain. We'd get UBI on top of other welfare measures.