The interest on all current money being borrowed by the US and other developed countries is effectively 0% because the Federal Reserve and other central banks have been creating all the money needed to cover it. Indeed, Japan has been financing its own deficit through its central bank for decades with no obvious consequences. As in Japan’s case, since the US Government owns its central bank it is highly unlikely the bonds the central bank is purchasing will ever be cashed in. They will instead be held on its books permanently like much of the debt from WWII, which means effectively forgiven. Regardless, even if the Fed demanded payment and interest was owing any interest would be considered profit and therefore would simply be treated as revenue, so even if the interest rate was somewhere just north of 0% officially it’s still technically 0. So, all this “borrowing” is not really an issue. Austerity would be far more expensive in both real dollar and real human terms in both the short and long run.