At the most basic level the function of a currency is: I gave you this amount of goods and services today, you give me a ticket that's good for goods and services from you (or others) in the future. Ultimately those obligations need to gradually decay away to nothing if they're unused, just to stop someone hoarding for ages and then causing chaos when they call in their debts. "Hey, remember those 100 chickens you got from me 5 years ago, you owe me 90 chickens now" seems a lot more socially sustainable over the long term than the reverse.