So...why not?

ACA-like premium tax credits could be used to ease the cost for the less well off. Medicare would get a pool of younger, probably healthier people.

The usual reasons are:

1) Adverse selection. Any time you have an "opt-in" insurance program, you run the risk of adversely selecting your enrollees. The basic principle is that anyone who has insurance that costs more than the Medicare premium (or who lacks insurance but has health costs higher than the Medicare premium) faces an economic incentive to enroll. Meanwhile, people on the other side of that equation (folks who have low health care costs or have low private insurance premia) will stay where they are. So you would expect many, if not most, of the new enrollees to be ones where actual health care costs are higher than the premiums they're paying. The proposal tries to protect against that by mentioning "actuarily fair" premiums, but that's never going to be done with individuals ('cause we don't do that anymore) and leads to...

2) Higher Medicare administrative costs. Medicare doesn't evaluate Medicare Advantage plans the same way that private insurers evaluate the coverage cots of large-group employer health plans. They don't do what you need to do in order to make sure that the premium charged will cover the actual health costs of the group. Medicare would need to take on a completely different administrative function - and one of the ones that drives up the administrative load of private plans relative to government plans in the first place.

3) Slashing provider compensation. We talked about this in the other thread. One key difference between private insurance and Medicare is that Medicare pays less for the same services. You'd basically be cutting provider compensation in half for hospital-provided services, and about a third for physician based care. That would lead to a nasty set of consequences, with many (if not most) hospitals with marginal budget situations going out of business (especially in rural communities) and dramatically increased pressure on doctors to leave Medicare altogether in order to avoid getting stuck with vastly lower income, leading to....

4) Upward pressure on Medicare rates. Government already has very little ability to control spending on Medicare - our per capita spending on seniors is double that of other countries in the same way that our per capita spending on non-senior health care spending is double that of other countries. When more services get funneled into the Medicare program, and if you start to lose providers and hospitals due to the lower reimbursement under existing rates, then you're going to have hitherto unseen political pressure on Congress to raise Medicare reimbursement rates so that hospitals and rural physicians don't go out of business.

Those are some of the main ones. And those don't even get into all the political concerns - like whether a Democratic-led government would deliberately underprice the Medicare premium below cost in order to drive private insurers out of business and de facto move us to single payer quickly rather than gradually.