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.
In your story RC incurs no opportunity cost for planting seed in and tending a less efficient field. There should be an interest rate as a function for lending the last Nth percent of the seed based on the opportunity cost of planting and harvesting the less efficient field, which at some point crosses 0 and becomes negative. The interest rate drops even more quickly once his next expected yield will be more than he can eat or store for more than a single planting season.
If RC is currently in the situation where his desired interest rate is still positiv...
I posted a stupid question a couple of weeks ago and got some good feedback.
@ChristianKl suggested that I start building a model of hypercapitalism for people to play with. I have the first one ready! It isn't quite to the point where people can start submitting bots to play in the economy, but I think it shows that the idea is worth more thought.
Analysis:
http://www.hypercapital.info/news/2015/4/19/a-published-model-of-hypercapitalism
Runnable Code - fork it and mess around with it:
http://runnable.com/VTBkszswv6lIdEFR/hypercapitalism-sample-economy-for-node-js-and-hello-world
I'd love some more feedback and opinions.
A couple of other things for context:
hypercapital.info - all about hypercapitalism
Overcoming bias about our money
Information Theory and the Economy