Look up the definition of the word FIAT.
It does not mean "unbacked".
It means by DECREE.
People are adopting Bitcoin voluntarily, not because some governement are forcing/coercing people to use it to pay taxes or transact.
Golds value is a fraction of what it trades for if it doesnt have the monetary premium it currently enjoys. Its a rock. It has uses for sure, but not worth the price its currently trading for.
Prices are set at the margin. If people, voluntarily, believe something has value, its fungible, divisible, can transport value through time and over distance, it is money.
All money is, is a ledger. A representation of the value you have exchanged with someone, that is fungible and allows you to trade with someone else for something you want/need.