There's a much-touted correlation between increased living standards / productivity and lower birthrates.
But there's a lag of about two generations.
Which is about the duration required for parasites, loan sharks and bureaucrats to infest and soak up all the productivity gains and tip quality of life and financial security downwards again.
David Ricardo observed circa 1800 that people of all social classes only settled down and married once they had the same financial security that their parents had.
Still true, except for some very religious communities.