These are all interesting economic academic exercises that are being thrown about.
The primary problem is this. We have in this country a health care insurance system that traffics in billions and billions of dollars annually. That system is not going to let Warren, Bernie, Joe or anyone else just declare "yeah, we're not going to do health insurance anymore in this country."
I posted this awhile back, but the only way you will get M4A in this country is through a form of "medicare creep." You just have to keep adding to those eligible, by age or some other classification, until the market for health insurance slowly dries up like the market for transistor radios.