They can be postponed so long as governments have easy access to market finance. But market access can dry up, as experienced during this crisis by some countries, where potential creditors have come to doubt that they will be repaid. Such doubts can either pertain to the economic ability of governments to stay current on their financial obligations.
In other words, market participants may conclude that the public sector is on the wrong side of the “Laffer curve”, so that a further tightening of the fiscal stance would be counterproductive. Or, they doubt the political ability of governments (let alone their willingness) to service their debts in view of political constraints.