I think there are too many people in America. I think the trend towards not having any is a response to that. That doesn't stop the 3rd worlders because life in an overcrowded America is still way better than their 3rd world shithole. It's just a lag, though. The grandchildren, if not the children, of those already here will delay their childbearing. Of course, there may be new immigrants to replace them, in turn.

Just because not every inch is paved yet doesn't mean that we aren't overcrowded. The population has reached a point where our level of technology is not permitting increasing quality of life at these population levels. Deurbanization could help.