The assets don't disappear. Gold doesn't disappear. People will still want to trade. They'll have assets and want different assets. Barter stinks. Countries will have surplus grain but want, say, tech. They can use a stable currency, say the Swiss Franc. Or they can use a scarce resource with a historical use as a currency, like gold.

And defaulting doesn't mean you get off, scott free. When you default and go bankrupt, there's an effort to recoup losses. Germany was encumbered with huge debt from World War One and went into full hyperinflation and eventually paid it off... in 2010.

Please, read that essay. It's fascinating and talks about things much better than I can.