From my experience working at a private insurer, (1) is, unfortunately for the body politic, more complicated than meets the eye.
The payor mix narrative has attractive qualities: it makes the private insurance companies sound like they're subsidizing care, the logical implication is that if we made these atlases shrug with M4A, the local PCP would go bankrupt. In reality, the narrative is at best reductionist and at worst misleading. But it rhymes with the truth, so it has a lot of mileage.
Private insurance reimbursements are negotiated in multiples of medicare because insurers and hospital groups alike expend tremendous amounts of money each year in administrative costs to challenge claims back and forth.
A surprising proportion of private insurance payouts are never reimbursed at the negotiated price. But hospital groups are OK with this, because it allows them to deduct the difference as a loss. Accountants rejoice!
Medicare/Medicaid prices are "low" because (1) the government has a strong bargaining position, (2) the population served by those fees has confounding variables of age and disability which raises the relative price of care (3) the government does not play claim-challenging games anywhere close to the extent of private insurers.
Private insurance companies and big hospital groups benefit the most from the status quo, and have the most to lose from medicare for all. But it would not be doom and gloom.
Medicare can adjust their prices to match the new population and tax base, hospital groups can stop running mini debt collection agencies, etc. There are real cost savings in M4A.
I think what's most likely to happen is an expansion of medicare advantage, (ma4a?). Since it's a program that effectively nationalizes the existing regime of private insurance.