Zero? Doesn't seem a very good faith assessment of the concept proposed.

yeah, obviously they were adults when their biological kids were raised and even if the weren't the primary, they surely would be hands on most of the time.

What does it change from parents with zero experience taking care of their children ?

Parents can ask their own parents for guidance. In the proposed scenario, the grandparents would have no one to ask because the previous generation is most likely dead already.

Wait the people that raised the kids that then had kids have no experience raising kids?

No, you misunderstood completely. Under the OP’s proposal, you have people raising their grandkids, not their kids.

So if I understand: Starting today, a pair of 20-somethings has kids, hands them off to their parents to raise - OK, their parents had experience now they are doing double-duty, but they know what they're doing raising kids.

Now... 20 years later, those children have kids and hand them off to their biological parents who ... have no experience raising children. Also, which grandparents? Mom's or dad's?

Ask the OP, I wasn’t the one who suggested the idea.

I think you’re missing the point (but possible I am).

My understanding is that OP is proposing that since the grandparents would be raising the kids, this would incentivize people to have kids much younger, e.g. in their early 20s, or even teens.

So the “grandparents” would be in their early/mid 40s, or even late 30s.

Definitely not “old people”.

And zero experience parenting, yes (just like any new parent today), but definitely more life experience, and likely more mature.

Not saying I’d advocate for this — I think there are still some flaws with this — but an interesting hypothetical that’s fun to discuss.