So, if government can’t make people have children they don’t want and can’t simply ship them in, Last asks if they could help people get the children they do want. As children go on to be taxpayers, government could cut social security taxes for those with more children and make people without children pay for what they’re not supporting. (Although you’d want to make sure there was no net burden of those children across their lives, as they’ll be old people one day too. There are limits to how far you could take that Ponzi scheme.)
Keep in mind that lower birth rates are an international phenomenon, so I am reluctant to place much weight on U.S.-specific factors. My sense is that the decline in birth rates is correlated with, if not caused by, increased education of women. If that is the main causal factor, then it probably is not something that is going to be reversed.
Also, I am not convinced that there is such a down side to slower population growth and eventual decline. Yes, it messes up entitlement programs for the elderly, but that is because those programs are ill conceived, particularly in not indexing the age of government dependency to longevity. You should fix the entitlement programs to deal with the demography rather than try to fix demography to deal with entitlement programs.