The brief window of opportunity we had of having a single-payer system on par with the National Health Service in Great Britain was in the immediate post-war years. We missed it. And there is nothing to suggest that such a window will open again in the immediate future, not with the suspicious -almost paranoid- attitude of the American people toward their government (if you were a census worker or with the civil service, you'd know), which hasn't improved after six years of this administration.
Considering how painful and difficult it was to pass Obamacare, not to mention its lingering unpopularity, introducing single-payer would meet an even worse reception from the American people, already suspicious of government overreach with Obamacare.
Single-Payer is dead on arrival. Count on it.
I can't say I entirely disagree with you. Wholesale transformation of the American healthcare system in one bill is virtually impossible. I was never someone that ever believed we could get single-payer passed in 2009 or 2010. However, I am one that believes it can be accomplished piecemeal. As I said above, lowering the age of Medicare eligibility to 55 would have been a huge accomplishment. I was also one that was strongly supportive of a public option. Our main problem was Joe Lieberman (and Ben Nelson, to a lesser extent). And, if it weren't for SCOTUS, a lot more people would be on Medicaid (even more if we had passed the House bill instead of the Senate bill).
As for the past, there were far too many missed opportunities. I would have happily taken Nixon's proposal. It wasn't perfect, but it would've been a huge foundation to build upon.