We are having less children because we are wealthy. Otherwise, you are making the argument that the birthrate in Africa is because of economic opportunity when in fact it is the lack of economic opportunity that causes higher birthrates. As intentionally child free, I even see this in myself. I am child free because I can think of all the opportunity I have that I don't want to give up. While if I had no opportunity, I would want to have a child who I could raise to do better economically than myself because I don't see any opportunity in my own lifetime.