One Grows Exponentially, the Other Does Not
An argument against interest that needs no theology. Debt compounds. The real economy does not. Run the two curves long enough and only one outcome exists.
There is an argument against interest that does not require you to believe anything at all. No theology, no ethics, no view about usury. Only arithmetic.
Debt at compound interest grows exponentially. The real economy, made of human labour and physical resources, grows linearly, and not even reliably.
Run two such curves against each other for long enough and there is exactly one outcome. The exponential one wins. Not might. Must.
Which means a financial system built on guaranteed compounding returns is not risky in the ordinary sense of possibly going wrong. It is a structure that arrives at a fixed destination, and the only open question is the date.
The standard reply is that growth compounds too, so the curves can stay matched.
Over short runs that is true, and it is how the system has kept moving. Over long ones it fails, because compound interest has no ceiling and production does. Output is bounded by people, energy, materials and time, all of which are finite and several of which are already straining. A rate of return does not answer to any of that. It is a number in a contract.