So if we're doing anecdotes...

One of our parents pays about $20k/yr all in for ACA - $12k/yr of premiums and $8k/yr on top of that (all unsubsidized)

Her (also unsubsidized) Medicare would be $6.5k/yr partA premium + $1.6k partA deductible+ $2.3k partB + $1k partD + $5k medigap, or about $16.5k total. She has no work credits for Medicare subsidy.

If you have subsidy from free partA premiums, then Medicare is about twice as cheap as unsubsidized ACA, yes. If you don't have subsidy for either, it's a little cheaper, but not by a ton.

So if you just stuck kids on existing Medicare pricing with no work credit for partA, then it would not be substantially cheaper than unsubsized ACA.

GP is pointing out that Medicare costs for the individual are about the same as ACA or cobra. It's not cheaper, unless you have the work credits to cut down on part A premiums.

So Medicare as-is for kids wouldn't be significantly cheaper than ACA for kids.

To make it cheaper, you'd need to either substantially increase the subsidy on Medicare, or decrease US medical costs (administration costs, drug costs, doctor salaries, etc)

Commercial health insurance is more expensive than Medicare because original Medicare Part A/B doesn't cover prescription drugs, and because commercial plans effectively provide a hidden cross subsidy to Medicare. Under a "Medicare for All" scheme, people would still need to purchase prescription drug coverage (Part D). And because Medicare reimburses providers at very low rates (often below their costs) they have to make up the difference by charging commercial health plans more.