The various government agencies in the US currently provide coverage for half of all citizens via Medicare, Medicaid, VA, and other federal and state programs. It isn't breaking any new ground to expand that to include all citizens. In fact, those agencies do much better at controlling healthcare costs than private insurance does because their administrative costs are much lower. This is largely due to less executive compensation and no agent commissions or advertising.

However, the one sure way to scuttle a single payer system here would be to attempt to "pay for" it as every politician so far has advocated for, with taxation or fees. One does not pay for things in the private sector by removing money from the private sector, and healthcare is big enough in America to do serious damage to the economy if done wrong.

As you alluded to, there are many people who would be involuntarily unemployed by single payer, and not just those who work for insurance companies. There would also be many who work for healthcare providers who now navigate the complexities of insurance to facilitate payments. That number is easily equal to the payroll of the insurance companies. Some economists have estimated the total numbers at around a million workers.

The federal government, specifically congress, has the distinct advantage of being the monopoly issuer of the currency that is only restrained in spending by the resources available that spending is meant to deploy. It needs no "pay for" balance to that spending, so it could provide coverage for the remaining half of the population without collecting premiums or additional taxes. It would be best if no such effort to collect its cost was made and the savings of the private sector was used to expand the economy to make jobs for the displaced workers.