I Bonds are an exception since everyone is capped at $10K and the rate changes quarterly based on inflation. BlockFi and other DeFi platforms were offering 8%, but had a minimum balance needed ($100K in most cases).
I bonds are subsidized by the US Treasury (this is why there’s a purchase limit), which has never defaulted on its debt.
Technically speaking, yes, though the USA is not invulnerable : for instance the dollar has lost a third of its value in the decade after the Nixon shock...