[kyrsey]

Glass also has decorative value. There's no easily imaginable risk of everyone suddenly deciding that it has NO decorative value.

So what? That's not the relevant risk. The relevant risk is that whatever you are using as a currency will DECREASE in value. Say, from the level of something highly valued as a medium of exchange accepted everywhere, to the level of a mere decoration. Or from a peak value where you bought in big during a speculative bubble, to a lower value where (yes) it's still worth something, but worth less than if you had kept it in treasury bonds or an index fund or a farm.