At this point in your hypothetical, if we're still talking about the US dollar, you'll have a lot more to worry about than generating a return on your investment.
If you think this is a serious potential outcome, then I suggest you forget about investing all together and instead start learning subsistence farming practices.
Since the US can print money to pay its debts you technically can’t “lose” in dollar-denominated bonds.
But you can still lose to inflation, collapse of the US, etc. The term “risk-free rate” is used as a baseline in comparing other financial assets; there are still risks involved and for those above a certain wealth value, spending some percentage to mitigate those may be intelligent.
Dual passports and foreign holdings are often involved.
