What I am going to say definitely does not apply to current USA but there are also downsides tohl having no debt. If a countries economy outgrows the debt the debt shrinks and results are positive. This is not hard in when a country doesn’t have something weighting it down like covid or right wing populists. Countries also want to own each others’ debt, so they can use their currencies whenever needed. When USA was predicted to wipe off its national debt it was optimal to keep some of it for those reasons and to also keep USD the main world trade currency.