There is a lot of "supposing" on Krugman's part. He does not even talk about the boomer situation, the lessening of the labor pool or anything regarding hard facts. He does suggest that we socialize medicine so that:
It's not at all hard to imagine that improving the incentives to focus on medically effective care could limit cost growth to well below what the CBO is projecting, even now.
The extent of his analysis is to imagine. To imagine that we focus on medically effective care, trigger words meaning rationing of medical care. We need to get rid of as many boomers as we can so that we are not spending so much. There, I just decoded Krugman for you.
Assuming nothing changes, SS will be spending more than it brings in in just a few years. Taxes will necessarily have to rise because now the general budget will be paying for SS benefits. Or general budget spending will have to decrease. Or, age of benefit will have to rise. Those are the choices. Krugman's thesis that we don't have to do anything is wishful thinking. He is wrong and common sense tells anyone with thought process that this is true.
If we do nothing, SS and Medicare spending will come out of the general budget. If that happens, we either just call it debt, or we raise taxes to pay for it. Or to keep SS solvent, SS taxes will have to rise either by rate increase or by salary increase or raise the age of retirement. Isn't it 72 right now? Seems high really. These are the ways to keep SS alive.
Doing nothing just raises the debt. Krugman is probably the last person you want to go to for economic predictions. His win-loss record is pretty low.
mspart