The bolded bit is my objection. It is very hard to put together the necessary political will, but it’s not impossible.

Calling it impossible is a call to not even begin to make the effort. Recognizing that it is very hard says that it will take a lot of effort over a long time to accomplish it.

I disagree with how you're reading my post. I've tried to be very clear that none of this is impossible in the sense of being against the laws of physics. It's just not possible to pass a single-payer health care proposal under the current circumstances. It's not a matter of "political will" - collective action problems and human psychology exist. The design of our political system expresses the political will of the voters in a certain way, and it very effectively measures the intensity of political will in addition to breadth of political intensity.

Recognizing that it's impossible to just go out and pass single-payer as long as we have the current political system and size of health care sector isn't the same as a "call to not make the effort." It's a call to not waste effort trying to pass a single-payer system now. You can't do it just by electing different people to Congress. Telling voters that you can, and then failing, and then blaming the dastardly insurance companies doesn't bring you any closer to actually passing it. The "lot of effort over a long time" can't work just trying to pass it through our current system, IMHO.

If we switched the US from the current system to a Parliamentary system, could we pass single payer? I think so. If we revised the First Amendment to limit campaign processes to those of European countries, would it be more possible? Perhaps. I think those types of proposals are also exceedingly unlikely to get passed (would require a lot of effort over a long time), but are more likely than single payer.