The welfare state's death spiral is this: Almost anything governments might do with their budgets threatens to make matters worse by slowing the economy or triggering a recession. By allowing deficits to balloon, they risk a financial crisis as investors one day -- no one knows when -- doubt governments' ability to service their debts and, as with Greece, refuse to lend except at exorbitant rates. Cutting welfare benefits or raising taxes all would, at least temporarily, weaken the economy. Perversely, that would make paying the remaining benefits harder.
Greece illustrates the bind.
If only a few countries faced these problems, the solution would be easy. Unlucky countries would trim budgets and resume growth by exporting to healthier nations. But developed countries represent about half the world economy; most have overcommitted welfare states.