If Obamacare's insurance reforms break the market, that calculus still won't change: Most people will still have insurance they like, and they will not be willing to give it up in order to solve problems in the individual market -- which now covers about 5 percent of the population and is expected to ultimately cover something over that. Even if the individual market functionally disappears, most people will still be covered, and most politicians will be unwilling to endorse a program that takes away what they have. There is no path to single payer from even a spectacular Obamacare implosion -- for the same reason that there was no path to single payer before Obamacare was passed.
Ironically, single payer seems much more plausible if the system succeeds. One possible path along which the health-care law could develop is that more and more employers dump folks onto the exchanges, breaking the link between employment and insurance for millions of Americans. If that happens but other problems remain -- such as rising premiums -- then you can imagine a series of reforms that ultimately leads to single payer, probably starting with a public option. Employers would probably still provide supplemental health insurance as a benefit, the way some do in the U.K., but it would be a relatively cheap add-on, not a huge portion of your compensation package.
So dash your hopes and allay your fears. An Obamacare failure would be bad in many ways, and it would mean significant changes for the insurance market. But we're not getting the National Health Service anytime soon.