I don't know why that is the default conclusion. It is much more likely that we live with a broken Obamacare system for a few decades that slowly unwinds into disaster. Disaster should rarely be celebrated.
Obama had one of the strongest political mandates since Reagan and even he couldn't do any better than Obamacare, and just getting Obamacare broke his political momentum (his plans to fix social security or immigration afterwards were shot by the Obamacare implementation). I don't see Hillary getting a bigger political mandate, she might not even have the gift of a Democratic Congress. Plus even if the Democrats do win Congress she might want a different legacy than Obama (whose legacy is tied to Obamacare).
Honestly I don't see why so many people want a US single payer setup so badly. Such a system would quickly push our healthcare system in a direction of inequality only seen in third world countries. The good doctors will go cash only, so the rich will still get the best healthcare (and will be the only ones to get it). The middle class can access those top tier doctors today via a (non exchange) health insurance plan, but once every service/test pays Medicaid reimbursement rates many doctors will opt of out taking whatever the government plan is. Already we have a third of doctors
not taking Medicaid, I don't see how the average person's health care improves when that number is 40%+ for a single payer plan. Especially when the only doctor we can get under the single payer plan is the guy who is the reason malpractice insurance exists and the wait time to see him is measured in months because of the backlog.
Seems like the best case scenario practically is the exchanges die from lack of use and we just roll back into a sort of pre-Obamacare system (with the worst parts like pre-existing conditions being fixed by Obamacare).