Soft money is a contract with the state, hard money doesn't expire. A $1 silver dollar coin from 1901 is worth a heck of a lot more than a $1 bill from the same time.
The notion of "hard money" is a joke.
Why? I don't even think modern Keynesians would argue against commodities this confidently, maybe MMT's would?
Hard money is a contract with generalized civilization that only has value so long as civilization continues to organize and complexify our resources. A $1 silver dollar coin still has a lot of objective value because civilization still provides lots of good and useful stuff for us to consume.
I fully agree that the value of silver is very unlikely to crater but it's not exactly Star Trek unlikely.