There's no need to assume there will be a positive interest rate on credit. Competition and automation (decentralized finance) may very well drive nominal interest rates to zero or negative by factoring in expected Bitcoin appreciation.

If BTC is the unit of account for the world, then it would generally increase in value at about the level of productivity growth of the world. Say, for example, 4% per year. So, 96 sats 1 year from now will be equivalent to 100 sats today in purchasing power. As a creditor, you might take the expected appreciation of the asset into account. If you want receive 5% real return on your value (for example), you charge a 1% nominal interest rate (5% return - 4% expected appreciation of the Bitcoin).

You can see how this may even result in a debtor paying a negative interest rate on credit, while still resulting in positive return on real value for the creditor.