The nuclear family concept of child rearing is awful. Humans did not evolve to raise kids in isolation from the extended family and the other adults of the tribe.
What's the alternative? Just have everybody constantly work and "time share" the kids between peoples shifts?
The nuclear family concept has its pros, it's also far from "raising kids in isolation", pros which these days are marginalized in favor of "alternative family concepts", but what all of them fail to address is the original reason why the nuclear family fell apart: Middle-class jobs can't support single income families anymore.
That is the actual problem, not in what constellations people chose to parent their kids.
As the economic pressure increases, more people from the family are forced into more work, leaving less time for the actual family and taking care of children.
In an ideal world work-load would have shifted, so both partners only work like half the time, leaving both of them time for family stuff. But that's not what happened, nowadays often both parents work full-time jobs and still make not enough to be able to afford a nanny, which would be an even worse solution.
But that does also not deal with the actual problem, it's just outsourcing parenting due to both parents having to work so much.
Imho that's the actual problem, solve that and you will free up more times for parents so the question of "What to do with the kids?" doesn't even come up in the first place.
I think the phraseology you've chosen here is confusing. Agreed 100% that multi-generational upbringing is the natural and ideal style, but I don't think that "nuclear family concept" is necessarily closed off or exclusive to other extended family members. Multi-generational living involves several "nuclear families" all clung together. It doesn't oppose the concept.
