I feel like it's apparent why comparing healthcare in the US with healthcare in, well, any of the aforementioned countries, is problematic. If you look at Western Europe (a bit less populous than the US) or the EU as a whole (a bit more population than the US) they have functional healthcare at much more reasonable rates than the US. Scale matters, but scale is also a matter of division. If a single system can't serve 300+ million people then it can be broken down into regional systems or state systems. That being said medicare already serves something like 60 million people. I'd argue that scaling a system to support 5x the number it currently serves is significantly more doable than scaling anything from zero.