Fundamentally the money is protected by the stability and reliability of the US government. Why have these private intermediaries in the middle at all?
If you want to sock away some cash in a place where it won’t disappear due to financial system shenanigans you should be able to give it directly to a US government entity like, say the USPS. Ultimately money is only worth anything in the context of a stable and reliable government anyway.
What you’re getting at is known as the “risk free rate,” or the risk premium of leaving your money with another entity for a period of time. US gov’t treasuries carry a risk free rate; you’d just buy treasuries.
If you want to sock away some cash in a place where it won’t disappear due to financial system shenanigans you should be able to give it directly to a US government entity like, say the USPS. Ultimately money is only worth anything in the context of a stable and reliable government anyway.