At the time gold and silver were the preferred store of value and unit of account and thus designated as a medium of exchange; i.e., legal tender. They didn't have a demand for gold as a contact material in electronics, nor for silver for photography. These demands alter the VALUE of gold and silver, do they not? Were substitutes found for gold and silver for industrial uses, would their VALUE fall? Are gold and silver then a stable store of value?
There is no such thing as a perfect currency. Indeed perfect distribution of information, zero transaction cost, and fluidity in all markets would theoretically eliminate the need for money entirely because all goods would be equally fungible.