This is a problem that'll solve itself eventually; the desire to have children is to some degree inheritable, so the people with genes that make them still likely to have children in modern society will outbreed the people with genes that make them unlikely to have children in modern society. Similarly the cultures that make people more likely to have children will outspread those that make people less likely to have children, so maybe in a few centuries the Amish, Orthodox Jews and Muslims will rule the earth.