"The income of a household is time-limited, whereas the state goes on forever (barring occupation by a foreign power). A government can simply roll over a debt ad infinitum. The UK has debts that date back to the South Sea bubble of 1720, and it is only worth repaying them if interest rates fall below historical levels which for very old debt has become the case recently. In 2015, George Osborne proudly announced how he would be paying back some one hundred year WWI debt, but this only made sense because the economy was doing so badly that interest rates were lower than when the debt was issued."
http://www.taxresearch.org.uk/Blog/2018/09/07/the-most-important-piece-of-economics-that-you-should-know/