That could years, decades, centuries, etc. waiting for that is not a strategy. If they do fail then every major fiat currency can (and may very well collapse).
The dollar is the underlying foundation of the global financial system since WW2. No if’s and or butts. Collapse of US dollar will spread unprecedented biblical financial contagion across every country including weaker currencies like the Rial and not just heavily indebted currencies like the Yen or the Euro or Pound. Bank runs will be rampant.
It wouldn’t be pretty. If people lose faith in a piece of paper whose whole worth is tied to faith the government will pay it back…..and then it turns out said government can’t? Oh boy.
A bit different than when Rome collapsed, British empire collapsed, Persian, etc. everything was based on a gold and copper currency system and wasn’t as highly integrated like today. You knew what you had in your hand had held value for hundreds if not thousands of years before you.
Currently the Paper in your hand only has value as long as society deems it has value. The second that is lost…..better hope you got bullets instead of paper.