If I were to give someone a Bitcoin loan, what interest rate should I charge? The massive uncertainty of that question is enough to discourage any such activity. I wouldn't make a Bitcoin loan with a one-week period.
Why? I can just hang on to it for the same return and no risk at all.
This is why BTC loans really don't make sense, to me. Positive interest rates are punitive if the value keeps rising, negative ones pointless.