I think you're confusing human/evolutionary nature with things that are a product of our, very recent, hyper-individualist societies. You ever heard the phrase, "it takes a village"? Early humans, heck even 50 years ago humans, lived much more communally than we do today. Especially if you read about native american societies.

It's reasonable that a child could rely less on the parents in their home being in any specific arrangement if there is a robust and wholesome community/found family for them to fall back on, which teaches them how to be a productive member and compels them to do so. Look at boys and girls clubs of america, as just one very modern example.

I don't think we are "meant to" have any particular relationship type or length, humans are far too diverse for that.

I concur.