Perhaps the peak romantic-cohabitation in the 1960s is the anomaly, and we're now retreating back towards something more sustainable. Consider that it is normal to live with parents in many other countries.
The normalness of living with parents correlates with the country's wealth per capita. Since most people in poorer countries cannot afford to own houses, they end up sharing houses their families own. Over time, the commonness of this occurrence becomes accepted as the norm.
It is NOT a good thing that the phenomenon is becoming common in America, because it indicates stagnating or declining wealth levels among the middle class.
Does collectivism correlate with a country's wealth per capita? Maybe it does!
The rationale goes that grandparents can look after the children while the parents are out earning money and in turn the parents can eventually take care of the aging grandparents as long as they don't need intensive care.
The argument being that this arrangement was historically the norm and worked out favourably for everyone compared to the modern "everyone for themselves" model where overworked parents neglect their children and the elderly can't age in grace because nobody has the time to look after them.
I'm not saying whether this argument is valid or realistic, but "let's rethink best practices" certainly isn't as exotic as you may think, even if this proposal seemingly runs counter to the individualism that is presently the cultural norm.
EDIT: The model projects in Germany also aren't cases of economic desperation either. These are middle-class families who think doing this is beneficial for their children. Maybe one should point out that "living together" in this case doesn't mean the same thing as "sharing an apartment" -- it's more of a "close neighbour" thing. So you could argue it's a bit hypocritical.
I don't think this arrangement was "historically the norm" for city dwellers. There, you had either nuclear families, or singles, or wealthy families complete with servants and maids.
In countryside, where you live is not important as long as you have land to grow food on and there's no drought.
But surely everybody in your family have their own idea what city they want to live in. It's sufficiently hard to negotiate this with your partner in nuclear family! Good look doing it with all those grandparents.
On what are you basing this?
What does bone the younger generation even mean and how does that manifest?
See my comments elsewhere in this thread. I have one and soon two older children living with us and the circumstances are in that comment. There are no excuses used in my house.
I don't see society saying kids need to live with their parents or saying they don't. I see individual families with circumstances that make the multi-generational living arrangements advantageous or even necessary. However it is not all positive. There are pluses and minuses.
Is there any reason to think that the romantic cohabitation trend of the 1960s might be unsustainable? Given how many people still dream of till-death-do-us-part, yes.