I agree, but I believe the primary reason is the effect of feminism on the economy over any cultural change.

Women in college leads to women in the workplace. Women in the workplace leads to surplus labor supply. That leads to reduced wages. The reduced wages then led to reduced birthrate.

Giving people tax breaks actually does lead to more children. The Nazis knew this which is part of why the native birth rates skyrocketed after Hitler implemented generous child-tax breaks for married couples