You're assuming Medicare, in its current incarnation, can properly scale like that.
I don't assume that, personally speaking. I assume that figuring out scaling issues would be part of the process of making it universal.
I don't expect, say, Bernie to jump into office, snap his fingers and then POOF we have m4a and a pony in every pot. There will be huge, huge amounts of fighting and conflict. Just like there was for Obamacare. It would be an ongoing fight.
But I expect that Bernie wouldn't roll over on the issue before the primaries -I expect he wouldn't roll over at all. I expect if he had to fight four years, he'd fight four years.
He isn't walking into that battle anticipating defeat.
That's my biggest problem with Warren; I see her handing over too much ground, far too early.