Doesn't default on what? A dollar isn't a claim on a percentage of the productive output of the USA, it's just a dollar. If dollars aren't worth anything, it's not worth anything.
People are not obliged to be paid in dollars. This means people could decide to be paid exclusively in some cryptocurrency.
So government collects taxes in dollars... what are they going to buy with that?
As far as real-world value, all that's needed is for the value to be "bootstrapped" if you will. When ethereum was first released all it took was for someone to say "I'll buy your 1 Eth for X dollars" and boom the value of Eth is bootstrapped because we can attach the value of an Eth coin to a real world item.
After that bootstrapping, the value of today's Eth is governed by supply/demand and the value it had yesterday.
Neither is it backed by the US military or the ability to pay taxes.
The fact that the dollar is in widespread use in a country that has a track record of stability, a well-managed economy, trusted institutions and a strong military all mean that if you want to put your faith into an arbitrary and made up asset, the US dollar is an excellent choice; but its value is a collective illusion nontheless.
The central banks of Austria, Bolivia, Brazil, China, France, Germany, Greece, Hungary, Peru, Poland, Venezuela, and Zimbabwe have all failed to stop hyperinflation.
Which of those were central banks of nations where debts including both private and public debt were denominated principally in fiat whose monetary policy was controlled by the central bank?
Some of them were pre-fiat (gold standard), and some were dealing with situations were obligations were largely in externally-controlled fiat (e.g., USD.)