The money needed to pay back the debt exists. In principle all debt could be repaid - BUT only if nobody saves money. That money which is saved cannot be used

The money needed to pay back the debt exists. 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. Some entity (the state) needs to ensure that nobody saves a lot of money in order for the system to work long term. 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.