Well, for one thing... China would be massively screwed, since they own the largest share of our debt.
That said, it would also destroy any remaining value to our dollar and make all imports (like oil, for one) not just expensive, but prohibitively expensive. It’d also end the dollar being thought of as the world’s ‘reserve’ currency, as no country would buy it.
Reserve currency? It is a fiction that the US Dollar is worth a dollar. There will not be a replacement for it, the sun will rise and the government will have to pay it’s debts. Your only kicking the can down the road by raising the debt.