Chris Bertram takes the hard stand that I think advocates of any sort of comprehensive single-payer plan, which Chris Bertram definitely is, are going to have to take: that some lives aren't worth saving.
In other worlds, what conditions would rational individuals not choose to buy insurance to secure treatment of? Here are Dworkin's answers: (1) "almost no-one would purchase insurance that would provide life-sustaining equipment once he had fallen into a persistent vegetative state .... (212). (2) nor would anyone buy insurance to provide expensive treatment for themselves in late-stage Alzheimer's (it would be better to spend the money in the here and now whilst you're fit and healthy). (3) people would also prefer to spend their money on their vigorous and healthy younger selves rather than on keeping themselves alive, at enormous expense, for a few additional months of low-quality life. So people wouldn't choose to spend thousands of dollars (or pounds) on insurance to buy expensive treatements to prolong the life of terminal cancer sufferers, or severe heart failure cases: people would rather spend the money on other things. And we can extend the thought to cover a lot of R&D too. It may be all very nice (stimulating, good for careers etc) for scientists and/or drug companies to devote billions to speculative research that might or might not issue in treatments extending the life of the terminally ill by a few month, but almost nobody faced with a choice between that investment of resources or spending the money on other stuff (education, culture, vacations, their kids) would squander their resources on such research.
So it turns out that the McMegans of this world are right about one thing: in a just society (not that they'd call it that) there would be less spent on expensive medical/drug research and development than a country like the US spends now. But that's a good thing : against a background of fairness and equality, rational and fully informed people would look at the opportunity cost of such activity and say "no thanks!"
I don't think this really follows. First of all, with the exception of cancer drugs, there aren't many drugs that are developed for people in the last few months of life. In other contexts, people complain about this: instead of developing a cure for cancer, we've got another goddamn antidepressant.
Expensive end-of-life interventions are stuff and procedures: high tech equipment, and doctors using that equipment to keep people alive. Hospice care, which specifically does not aim to keep people alive artificially, is cheaper than a hospital, but it's still not that cheap.
Furthermore, the things that keep elderly people alive also keep alive young people with years of high quality life ahead of them, if they can survive the crisis. People do come off of ventilators, heart-lung machines, and so forth. It might be worth having them for young people, and once we've got them, the marginal cost of using them on old people may be worth it.