Fellow medicare actuaries - question about bernie sanders

Okay one of my thoughts the other day was whether a single payer system might actually work if it was managed sufficiently.

As I don't work on Medicare products, I don't know the challenges and nuances currently facing Medicare as well as some of the actuaries on here might.

What if Bernie instead was pushing to expand Medicare Part C to everyone, essentially establishing a nationwide HMO network? Because it would be Part C, it would be a national health system administered by existing HMO network providers, with subsidization and risk adjustment through the federal government. It would hopefully avoid issues of over-utilization and medical trend.

Is any of what I am thinking out of line? Does anything begin to break-down? Where are the key pain points?

Thanks!