Civilization-ending? How is that even remotely in the realm of possibility?
The government finances its operations by issuing T-bills. If nobody is willing to buy T-bills, the government can't get cash to pay government employees, including the FBI, CIA, military, etc. It's unlikely that people are going to continue working if they don't get paid. Historically, governments that don't pay their soldiers, police, intelligence forces don't remain governments for long.
Although, I do need to add, while your scenario is technically possible, I do think it's vanishingly unlikely. The US has been down that road before and survived, as have many others.
You can have civilization-ending events that don't involve everyone dying - the fall of the Soviet Union in 1991 was one. It was still hugely disruptive to everyone who lived in the Eastern Bloc. The U.S. Civil War is another one, though plenty of people died then too. It's not the end of all civilization, just the civilization we're in.
If nobody is willing to buy T-bills, the Fed will just monetize the debt, which is something they have been doing since 2008.
There's a reason the Fed's balance sheet is now around 8 trillion dollars.