"we are currently retiring debt with borrowed money. We do it today!"
This called rolling over debt and has been done for decades if not centuries.
"The government would default."
Since the Federal govt has the power to create money, it cannot default by definition.
Yes, it can. To say otherwise is semantics.
It would be forced to renege on agreements by reducing the promised outlays. That is default.
This has been widely discussed, but never called what it is.
Congress hold the power of the purse. Sure, they could get the Fed to crank out additional bucks, flood the market and cause a inflationary spiral.
But our debt is largely foreign held, the inflation would devalue their assets, they would pull their money out.
This situation would be the great fear of everyone who has ever put tinfoil on their head and and claimed the end of the world was coming, because we came off the gold standard, and no longer backed our currency with anything but a promise.
A promise is dependent on fiscal soundness, a soundness lost if this situation were to occur.
Making new money was what caused the Carter disaster when interest rates hit 27%+ in real estate. (I was a Realtor then)
That was just a bump, and a warning.
What I am discussing here is way bigger than a bump. It is a cliff.