I think the big reason universal health care isn't viable in the US is because of the system we already have in place. Big insurance and high-cost health care are firmly entrenched, and it would take an act of god to change this without upsetting the entire economic apple cart. I'm not really sure what the answer is at this point, but I agree with you when you say it's not a viable long term solution.
Firstly, you're addressing a different question than everyone else (even though I agree it's the right question), because you're talking about political viability whereas most people get stuck arguing about economic viability.
Second, there are a whole bunch of pieces to the system that keeps this in place. Relationships between lobbyists and government. Campaign financing regulations. Gerrymandering. SCOTUS interpretation of the commerce clause. If you started fixing a few pieces like those ... one at a time over decades ... you could change a lot of things.
