Right, but it's not like M4A is the only path to some of those right? I mean of course high quality care is popular, but you don't need M4A for that. Same with decoupling employment and insurance. And yes, making things cheap/free is always popular. But again depending on framing: "Do you support eliminating deductibles and premiums by raising taxes on the middle class?" My answer to this is a resounding yes, but I think it would not be super popular. And you can come up with things that aren't popular about single payer too.
I guess it just feels like leftists (to use your framing from earlier) take as a given that M4A is the most "left" health care policy, and therefore it must be the best. And I just don't get that level of certainty. It takes some stones to say we are going to ignore the way that many wealthy countries are managing successful health care systems, we got this.
For me, it's not at all clear the M4A would be a better solution than something that would get portrayed in some circles as incremental change, but would in reality be a big fucking deal that could make a huge difference in many lives. I also think that M4A would be much harder to pass, much harder to implement, and much riskier if we fuck it up. So while I'm happy to concede that it could really be the best policy, I totally understand why a national party or a presidential candidate wouldn't want to endorse it right now.