The higher the standard of living, the lower the birth rate. So it goes in Europe, Japan, and in every first-world nation. In any given population, it’s always the poor who have a higher birth rate.
If America is wise (and less apt to listen to racist right-wing pundits), we’d be happy to open wide the floodgates of immigration. They work harder, commit less crime, and they’ll be the ones taking care of us baby boomers when we get old. Otherwise, we’ll wind up like Japan, where too many elderly are not receiving any personal help at all, and all too often die alone and forgotten.