But you can also write the opposite parable:

Crusoe has finished planting his most fertile fields and is preparing to plant his remaining seed in the less fertile adjoining land when the stranger arrives.

S: I have found some excellent fields, too far from your home here to be of any use to you, but which will serve me well; and I need only some seed. I see you have some excess, which will be much more productive in my fields than in yours; might I borrow it, and easily repay after harvest?

RC: How much seed will you repay me, for each I lend you now?

S: One seed for each; my religion would forbid any other arrangement.

RC: The land I intend to plant them in is not so lush, but even so I expect to reap many times the seed I plant. Where is my self-interest in placing it where it will yield less?

Moral: some goods decay, but others can compound themselves--or, more often, create other goods that can create other goods in a long chain that ends with more of the first good.

The role of money is to make such long chains implicit; a positive real interest rate reflects that such chains are possible for most goods, and any exceptions will increase in price faster than interest accumulates.