I haven't had time to get up to snuff on the studies (or actually I guess it's just "study") Megan McArdle and others cite in support of the notion that health insurance has no discernible effect on mortality. But there are heuristics one can use to evaluate their claims.
Another thing is that health care denialists* just don't appear to take their own claims seriously. Or at least not conspicuously so. For instance, McArdle has nowhere (as far as I know) suggested that she avoids the doctor or tries to go without health insurance. But if she believed what she said, she would have to think that health insurance and marginal medical costs would be just a tremendous waste.** Actually, it's worse than just a waste - overgenerous health insurance policies may lead to the overconsumption of a dangerous service. Like, everyone will be out there getting daily bone marrow biopsies or whatever, 'cause they're free. So she'd really want to avoid that sort of temptation.
There is also the fact that all of the denialists mentioned are themselves ardent fans of markets. Which, I admit, are awesome and magical things. (If only doctors had invisible hands like markets do!) But if the denialists' claims about the inertness of health care are to be believed, what does that say about markets? Most anyone who can afford health insurance purchases it. And almost everyone spends money at the margin for medical products and services. But on the denialists' claim, they could only be doing so out of an obstinate, irrational belief that medical products and services are a benefit to their health. (Theories based on signaling are bracketed out here as plainly wanting.) Yet instead of drawing what I would think to be the more salient skeptical conclusion about market rationality (namely, that it's a zombie idea), they draw the much narrower conclusion that public health coverage would be a waste. Interesting.
Finally, there's the fact that those arguing against public health care on the basis of health care denialism by and large are those ideologically committed to opposing tax-based redistributions of wealth and services to the less productive members of society. Though that's probably just a coincidence.
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* While the health effects of "health insurance" and health care are distinct issues, the guiding theory of the denialists (sometimes hidden) is in fact that medical care itself just doesn't do all that much. Leading libertarian health care denialist (but perhaps I repeat myself) Robin Hanson is quite explicit about the point:
For many decades health economists have known that the best available evidence shows little or no relation at the margin between med[ical care] and health. The health economists advising all the major sides have long known this. When the data is this noisy, there will always be exceptional studies, and as Megan says, the left prefers to cite exceptions that find more med tied to more health; the right prefers to avoid the issue.
These tactics are far from random accidents; neither side wants to contradict the US public, with their religious-level faith in the healing powers of medicine.
"Religious level faith in the healing powers of medicine." Ouch.
Then again, Robin himself is paying hundreds of dollars a year in premia to have his severed head frozen (sorry, "vitrified") until 2210 - when medicine will be able to extend his life for thousands of years. Because I guess health insurance was a little too speculative. (Wait - isn't cryonics actually a kind of health insurance?)
** That is, at least if the only thing that matters about health insurance is that it's supposed to increase life expectancy. (And if that's not the only thing that matters about health insurance, why is the effect on mortality leaped upon as if it were a proxy for the importance of public health care? [Mostly a rhetorical question, that last one.])
