United States government bonds, or Treasurys, have a tremendous influence across all tradeable markets, including Bitcoin (BTC) and Ether (ETH). In that sense, risk calculation in finance is relative, so every loan, mortgage and even cryptocurrency derivatives depends on the cost of capital attributed to U.S. dollars.
Assuming the worst-case scenario of the U.S. government eventually defaulting on its own debt, what happens to the families, businesses and countries holding those bonds? The lack of interest debt payments would likely cause a global shortage of U.S. dollars, triggering a cascading effect.