Similarly, contraceptives aren't the reason people have fewer children. It's the fact we can't afford childcare and housing for a family.
In other words, yes current state is a function of all past states, but recent history has a far higher influence on next state than the far past. At a certain point you have to assume its influence to be close to zero.
In my own family, the American half reproduced earlier and had many more children than the "old country" half, due to economic distress and war. People in my father's generation were not directly affected by World War II, but the economic contractions that gripped Europe in the 1970s really show when you look at the reproductive choices my "old country" family made compared to my US family (which supports your previous post).
It's just weird that this longer conversation posits either idealized 1950s suburban America or an individualist society. Especially as employment configurations evolve, we should be aware that those are not the only options.