People arent going to go to the store with gold and silver. Metals are a terrible medium of exchange which is why throughout history they are typically not used as the primary currency. Currencies should be a claim on asset and not the asset itself. Thus I can see a digital version of the gold and silver being the primary mediums of exchange. If I have a crypto currency that maintains the purchasing power of gold and entitles me to interests on my initial capital then  I would certainly use that over the physical asset.