The nuclear family lets itself to a village where extended family is nearby and children move between households to provide labor rather than paying the cost of slaves. (really, that’s the answer).
The nuclear family requires very cheap land. This is probably the most overlooked cost.
The benefit of the nuclear household and shared labor, is that (a) it’s possible for family capital to be mobile, (b) it prevents dysgenia from slavery, (c) delays childbirth, and (d) prevents dysgenia from underclass reproduction.
The nuclear family is also superior under early capitalism where we move people to capital. The problem is, it’s destructive.
Move capital to people rather than people to capital.
END IMMIGRATION OF LABOR.