One - perhaps the most - important claim which is at least implicit in the "serious" critiques of Sanders' health plan is just false: It is not the case that under Medicare, a patient cannot be denied a treatment he/she wants. Medicare coverage is still subject to a "medically necessary and reasonable" standard - the latter part of which officially and explicitly involved subjective judgments of cost vs. chance of success until 2008, and I find it incredibly implausible to believe that it doesn't still effectively and implicitly involve such judgments on a day-to-day basis, at least for patients in demographics which are unlikely to be able to challenge decisions in court.
To sharpen the critique, the challenge should be posed in the form: will denials of treatment increase under a Sanders-style single-payer plan?
The analyses (like Thorpe's - which doesn't hold up) and the, to put it bluntly, ramblings (like Krugman's and Klein's), which answer in the affirmative are relying on two assumptions, one much-more-probably-than-not false, and one false to a practical certainty: the assumption of significantly increased demand (not borne out by reviewing the data from other jurisdictions), and the assumption of non-significantly increased bargaining power over the costs of drugs and medical supplies (whether because such power is simply assumed - obviously falsely - not to exist under single-payer systems in wealthy countries, or because a significant drop in willingness to supply is assumed contrary to all international evidence. This is actually more of a theological statement, or perhaps a superstitious one, than anything else; "Ooh, ooh, The Market won't like this...it will desert us!"). Furthermore, it is at least worth contemplating the potential for a cultural change in patterns of healthcare consumption in the US - a shift in preferences such that consumption becomes less wasteful - following (with some delay, of course), the major institutional shift to single-payer.
To spell out where Sanders is fudging (it has nothing to do with logical possibility or arithmetic, though yes, the campaign document did contain a faulty estimate of drug cost savings which needed to be updated): he isn't saying what denials of treatment would look like under single-payer vs. what they are under Medicare (or under the US healthcare system as a totality) now. There's no way to make a sound and relatively fine-grained prediction about that at this point.
But there are good reasons - the ones just stated - to think that denials will actually go down - certainly on the whole (that is, if we include denials of treatment by all insurers, public and private, under the current system), but even with respect to just Medicare as it currently exists. They won't go to zero, but they aren't at zero now.
The US does ration healthcare; it just uses different mechanisms than Canada and Europe (places where, incidentally, private clinics also exist - that's one of the biggest and most incomprehensible whoppers going around - and so, just like in the States, if you have the money, you can have whatever you want done right away). Rationing in the US is achieved via high costs of care - and that includes plenty of types of care that are covered under Medicare, but for which the program does not reimburse 100%.
And that's what really bugs me - as a citizen and resident of a single-payer country - about what's coming from Krugman, Klein, and plenty of others, at this point. Their opposition to Sanders and favoring of Clinton are driving them to embrace one of the greatest, most thoroughly debunked, bogeymen of the Right: "You don't want healthcare like they have up in Canada - they RATION!!! [cue scary music]" Yes, as does the US (even more so if we don't merely and unjustifiably define rationing in a way which excludes the mechanisms by which this is achieved in the US); but Canada does it in a way which delivers objectively superior health outcomes to a much larger percentage of the population at a much lower cost.
Before this election, no self-respecting progressive would have taken this seriously, let alone disseminated it.