The problem there is that the existing Medicare system is predicated on paying less for the same things than private insurance and then letting high private insurance premiums subsidize the Medicare system. Which obviously doesn't work when everyone is on Medicare, so the cost per Medicare patient would go up at the same time as you're adding hundreds of millions of people to it.
So the Medicare tax would have to increase dramatically, but the Medicare tax is pretty regressive. Meanwhile a lot of companies would then get away with discontinuing their private insurance coverage without increasing employee compensation by the same amount. So you're basically talking about a huge transfer of wealth from the middle class to corporations.
We should probably look into some other method of dismantling the employer-provided healthcare system. Maybe something that involves primarily catastrophic coverage as a baseline.