A commenter elsewhere suggested that once the extended family disappeared fertility was doomed. The idea being that the costs of motherhood are so high that women will not accept those costs for more than one or at the most two children unless they have a strong support network.
This does sound like a plausible argument.
Perhaps humans have to have extended families in order to breed. Perhaps the nuclear family is a toxic idea.
If you want extended families there are several other things you need. Obviously you need stay-at-home mothers. You need stable decently paid employment for men, stable being the more important factor by far. And you need social stability. You need tight-knit organic communities of people who know and trust each other and those communities need to remain intact. Social mobility was one of the major factors that destroyed the extended family. People need to be able to find jobs in their own communities.
The Industrial Revolution undoubtedly played a rôle in disrupting established communities but the urban working classes managed to create new communities that were actually quite healthy. Those communities thrived until our leaders (encouraged by the corporate sector) decided to declare war on the working class.
Conservatives like to blame the welfare state for most social ills. In fact the wholesale destruction of manufacturing industry and the destruction of rural communities have been far more disastrous.
Of course the decline of religion and the growth of feminism have contributed to the demise of the extended family but the big factor is the breaking up of communities when children move to other cities to pursue their careers, or when family members are forced to leave established communities because they cannot find work.
The corporate sector loves the idea of a “flexible” workforce. In practice that means a demoralised workforce and a loss of community. These are both bonuses as far as the corporate sector is concerned. The corporate sector does not like extended families - people might start to think there was more to life than being a docile worker and a compliant consumer.
In fact if we are ever to rebuild real communities we need to realise that there really is more to life than working and consuming.
Real communities and extended families have other benefits aside from increased fertility. They provide an alternative to hedonism and degeneracy and they also provide some protection from evils such as feminism.
Of course it’s understandable that our politicians didn’t think of any of this. Who would ever have imagined that tight-knit communities might turn out to be essential for a healthy society?