Thing is, it can't be it's only function - because in order for something to be able to actually function as permission digital cash it needs to have at least some value to someone who is a net recipient of that cash (i.e. they manage to receive more of it than they intend to send today). That value proposition can be stupid, but it has to be there, people have to agree in a social consensus that it has some value.
You can have something whose primary function is cash between parties and a secondary, minority function is being melted down for jewelry - since that provides a backstop ensuring that it stays viable cash, that there is some value for it.
You can have something whose primary function is cash between parties and a secondary, minority function is used to pay taxes or tarriffs to some party (starting with quite ancient times) - since that provides a backstop ensuring that it stays viable cash as long as that tax collector stays relevant.
That minority use can be relatively small and rare, however, if it does not exist at all, then it's a poor medium for cash.
You can have something whose primary function is permissionless digital cash between parties and a secondary, minority function is a speculative investment bubble - which ensures that it stays viable cash as long as that bubble does not pop. But not really after that.
You can have something whose primary function is cash between parties and a secondary, minority function is being melted down for jewelry - since that provides a backstop ensuring that it stays viable cash, that there is some value for it.
You can have something whose primary function is cash between parties and a secondary, minority function is used to pay taxes or tarriffs to some party (starting with quite ancient times) - since that provides a backstop ensuring that it stays viable cash as long as that tax collector stays relevant.
That minority use can be relatively small and rare, however, if it does not exist at all, then it's a poor medium for cash.
You can have something whose primary function is permissionless digital cash between parties and a secondary, minority function is a speculative investment bubble - which ensures that it stays viable cash as long as that bubble does not pop. But not really after that.