The thing is, people generally do view currency as something concrete, and its value is largely based on faith in that concreteness. Once they stop having faith in that, it seems likely that they’ll stop valuing it.
And one of the key reasons for maintaining a positive interest rate on lending is to encourage sound investment decisions by the capitalists borrowing it. Low interest rates beget more and more speculative investments, and more and more of those will go bad. And then you get a debt crisis.