The money needed to pay back the debt exists.<p>In principle all debt could be repaid - BUT only if nobody saves money. That money which is saved cannot be used to repay debt. It is this money which requires new debt to pay back old debt.<p>Some entity (the state) needs to ensure that nobody saves a lot of money in order for the system to work long term.<p>A weak state would rather take on new debt on its own instead of taking the money out of the hands of its citizens. This creates a temporary relief but in the long run it creates an increasing amount of money saved which in turn worsens the problems.