Everyone knows Medicare isn't "free".

Medicare and COBRA are not similar costs. My parents pay half what I would pay if I took COBRA and they have a better plan. Neither of them were struggling before they retired and I'll put it this way, they bought a second home in retirement.

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.