Krugman points out that Medicare is indeed sustainable in its current form:
What is true is that the U.S. Medicare is expensive compared with, say, Canadian Medicare (yes, that’s what they call their system) or the French health care system (which is complicated, but largely single-payer in its essentials); that’s because Medicare American-style is very open-ended, reluctant to say no to paying for medically dubious procedures, and also fails to make use of its pricing power over drugs and other items.
So Medicare will have to start saying no; it will have to provide incentives to move away from fee for service, and so on and so forth. But such changes would not mean a fundamental change in the way Medicare works.
Of course, what the people who say things like “Medicare is unsustainable” usually mean is that it must be privatized, converted into a voucher system, whatever. The thing is, none of those changes would make the system more efficient — on the contrary.
So this business about Medicare in its present form being unsustainable sounds wise but is actually a stupid slogan. The solution to the future of Medicare is Medicare — smarter, less open-ended, but recognizably the same program.