More plausible explanation if you need a fanboy defense: Nothing in the text ever says the coins are pure gold and silver. They could work just like Muggle money and when people refer to them as silver and gold it isn’t any more meaningful than calling US pennies copper.
The fact that coins are stored in large vaults and apparently aren’t earning interest does indicate that there are aspects of the system that don’t work as the modern muggle system does, but there’s not a strong argument that the coins are pure metal.
More plausible explanation if you need a fanboy defense: Nothing in the text ever says the coins are pure gold and silver. They could work just like Muggle money and when people refer to them as silver and gold it isn’t any more meaningful than calling US pennies copper.
The fact that coins are stored in large vaults and apparently aren’t earning interest does indicate that there are aspects of the system that don’t work as the modern muggle system does, but there’s not a strong argument that the coins are pure metal.
Both possibilities (protective spells, not real gold) are suggested at the Wikia.