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.

Or you could buy Swiss bonds?

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.