Apparently Megan McArdle is not convinced that health insurance promotes health. I assume she (and any reasonable minded individual) would agree that death can be caused by lack of sufficiently good health. It is, therefore, only a trivial bit of logic to conclude that if insurance promotes health it can also be life preserving. Or, turning it around, if uninsurance leads to bad health outcomes it can also increase mortality.
That uninsurance is bad for you is easy to defend if you know the research. There is a large body of health services and health economics literature that documents the negative effects on health due to lack of insurance. My own work with Steve Pizer and Lisa Iezzoni, published in Health Affairs, reviews some of that literature as it pertains to individuals with chronic health conditions.
Using data from the National Health Interview Survey, a recent report found that 46.0 million nonelderly U.S. adults (ages 18–64) reported having at least one of seven major chronic conditions in 1997; by 2006, that number had risen to 57.7 million. This and other studies document much lower access to care among uninsured people with chronic conditions compared with insured people. Adverse access markers include lower rates of having a usual source of care, fewer primary care and specialist visits, more frequent use of emergency departments (EDs) for primary care, and difficulties affording services. Such studies complement a growing body of research documenting poorer health outcomes among uninsured people with chronic conditions. [3-6] Acquiring health insurance can improve people’s health and change downward trajectories of functional declines.
(Bold mine.) Since health outcomes pertaining to the transition to Medicare is one focus of Megan McArdle’s Atlantic Monthly piece (see also her related blog post; h/t Tyler Cowen), let’s focus on that for a moment. In Health of Previously Uninsured Adults after Acquiring Medicare Coverage  McWilliams, et al. find that
eligibility for Medicare coverage at age 65 years was associated with significant improvements in self reported health trends for previously uninsured adults relative to previously insured adults. … our findings suggest long-term benefits of gaining insurance on the health of previously uninsured Medicare beneficiaries, particularly those with cardiovascular disease or diabetes.
(Again, bold mine.) The evidence that insurance and the access to care it facilitates improves health, particularly for vulnerable populations (due to age or chronic illness, or both) is as close to an incontrovertible truth as one can find in social science.
 Hoffman C, Schwartz K. Eroding access among nonelderly U.S. adults with chronic conditions: ten years of change. Health Aff (Millwood). 2008;27(5):w340–8.
 Wilper AP, Woolhandler S, Lasser KE, McCormick D, Bor DH, Himmelstein DU. A national study of chronic disease prevalence and access to care in uninsured U.S. adults. Ann Intern Med. 2008;149(3):170–6.
 Ayanian JZ, Kohler BA, Abe T, Epstein AM. The relation between health insurance coverage and clinical outcomes among women with breast cancer. New Engl JMed. 1993;329(5):326–31.
 McWilliams JM, Zaslavsky AM, Meara E, Ayanian JZ. Health insurance coverage and mortality among the near-elderly. Health Aff (Millwood). 2004;23(4):223–33.
 Ayanian JZ, Zaslavsky AM,Weissman JS, Schneider EC, Ginsburg JA. Undiagnosed hypertension and hypercholesterolemia among uninsured and insured adults in the third National Health and Nutrition Examination Survey. Am J Public Health. 2003;93(12):2051–4.
 Fowler-Brown A, Corbie-Smith G, Garrett J, Lurie N. Risk of cardiovascular events and death—does insurance matter? J Gen InternMed. 2007;22(4):502–7.
 McWilliams JM, Meara E, Zaslavsky AM, Ayanian JZ. Health of previously uninsured adults after acquiring Medicare coverage. JAMA. 2007;298(24):2886–94.