but this actually remains remarkably stable over history. In Roman times you could buy a fine set of clothes for about an ounce of gold
But why?
In practical terms, gold generally speaking, isn't astoundingly useful in an large scale, even less so in modern life than in antiquity, it's not really that rare really is it. There's piles of gold to satisfy demand, I don't think any industry (semiconductors for example) has trouble buying gold if they need to buy some.
I think gold, and silver, and diamonds etc are valuable mainly because they are desirable things to have, and that's purely because people, for no good reason have decided they are desirable things to have.
Diamonds are an even bigger case in point, gem diamonds have essential zero practical use in any sort of scale that matters, and yet they are assigned by the people such fantastic monetary equivalence that they are used, much the same as bitcoin in fact, for exchanging value when you don't want to exchange money (ie, blackmarket). Industrial diamonds, the ones we actually use, have all the practical applications, but none of the value. Neither gem diamonds nor industrial ones are rare, hell we can manufacture them.
All it takes is for the people to decide that gold, silver, diamonds really are not "all that" and whoosh, there goes the market, there would be piles of supply to satisfy the actual users of them in practical applications for a long time.
Bitcoin isn't really that much different, in fact the parallels with diamonds especially are really close if you think about it - an artificially constrained supply of a not-rare thing, where the valuable type has little practical purpose (ie, bitcoin) and is valuable because people say it's valuable, and the non-valuable type has lots of practical purpose (eg various altcoins due to low fees/bigger blocks/segwit etc...) and probably should be more valuable because it's more useful but isn't more valuable because the people say it's not valuable.
People are strange.