There are many reasons people don’t have children (and I suspect the primary one is the desire to avoid the sacrifice and inconvenience), but it doesn’t change the result: If we don’t make a next generation of workers and CONSUMERS, our government will import one. It isn’t theory; it is playing out in front of our very eyes (and has been for decades now).
I’m sure a large part of the problem is a general reluctance to be inconvenienced, especially in urban areas, but I’m also sure there are still enough American couples who would prefer to have a larger family, large enough to deal with the problem.
Several countries - Russia, Hungary - have recognized the problem and are putting policies in place to rectify it. But such policies are the last thing the US government, as currently constituted, would consider.