Perhaps I am being a bit obtuse, but I think while I personally agree with your line of thought, the original thought is a hypothetical that could never happen anyway (we can't wind back time), so it's kind of a moot point in either case, as I'm fairly sure the OP didn't mean it like that and was just using it as a contrast to demonstrate to people how immigrants benefit from migration and are thankful and feel lucky to be in a country such as the US.