It’s not just a matter of having people. The main reason we care about the worker/retiree ratio is that workers generate a surplus that can be used to take care of retirees. To the extent that immigrants and their kids are able to quickly join the middle class that pays the taxes that supports retirees, then immigration helps. But that same fact limits how much you can rely on immigration to address lack of internal population growth.

(That’s why European efforts to combat their lack of population growth will fail. The US, by contrast, is far better at assimilating immigrants into the middle class and will be relatively more successful.)

> That’s why European efforts to combat their lack of population growth will fail. The US, by contrast, is far better at assimilating immigrants into the middle class and will be relatively more successful

Not sure the American Dream is still alive. The Trump administration might as well be the straw that broke the camel's back.