There are three basic choices. We can keep on just as we have in the past until the program collapses of its own weight. Or we can restrain costs by letting the federal government ration medical care. Some patients would have to wait months or years for procedures now taken for granted -- and some wouldn't get them at all. Death panels, anyone?
Or we can switch from a guaranteed-benefit program to a "premium support" model. Everyone enrolled in Medicare would be allotted a certain amount of money to buy a health insurance policy, with higher amounts for sickly patients and extra help for poor ones. Insurers would compete to get their business. A retiree who wants a policy costing more than the government payment would have to fork over the difference.
As the Congressional Budget Office notes, "most elderly people would pay more for their health care." That's not a terribly enticing prospect. But we might as well stop pretending there's any alternative.
Eventually, someone has to pay for all the health care the elderly get. In the past, the cost has been passed on to younger workers. As the senior population expands and the labor force fails to keep pace, that trick will no longer be feasible. When a Ponzi scheme runs out of victims, it ends in tears.
Critics can offer different solutions, but there is no escaping the constraints at work here. The reason people will dislike what Ryan offers is not that he's needlessly cruel. It's that his plan confronts reality, and reality bites.