On your last paragraph that's how human raised kids for the first 10k+ years. In small groups of multigenerational settings.
Only in the last 100 (maybe even less, say 60) or so years we've started to say fk that, when I retire I want to be on a beach and not taking care of my grandkids.
It seems like the experiment of retirement is a recent phenomenon and one that the jury is still out in terms of whether it's a good idea, let alone sustainable.
Before 100 years ago there was precious little nation-state level social security, so families were far more dependent upon other family members. Middle age parents needed to support their parents directly, so there was an obvious quid-pro-quo for the grandparent to take care of the child.
Our forefathers changed this dynamic by making this arrangement somewhat invisible, in that we pay social security tax instead. We still take care of our parents, but not directly, and it's by the gun of the tax man rather than by a family arrangement. There is little incentive for this kind of unspoken bargaining of the past where you may take care of shelter and the grandparents take care of a kid. There's an IRS agent waiting with a gun if you don't pay social security, so any unspoken bargaining power is gone and the working age parent basically gets the squeeze from both sides.
Only in the last 100 (maybe even less, say 60) or so years we've started to say fk that, when I retire I want to be on a beach and not taking care of my grandkids.