Well, I mean, I suppose you could manage one with nuclear families if you really wanted to, but you'd have to work at it. Short version: right now, people who have kids spend something like half their lives either as kids living with their parents, or raising their own kids, so it makes *some* sense that "parents with kids" is our basic societal unit (although even then, 20th nuclear families are the exception, not the rule.) But if you reduce the number of kids at any given time by 1/20 at the same time you greatly increase the lifespan, average people are going to be spending a tiny, tiny fraction of their lives in those setups, and it will be an rare and fleeting life stage rather than the standard.

I've seen way too many stories where people supposedly live thousands of years, and the population is fairly stable, and yet the majority of adults are still raising young kids.