No issue at first. Time value of money is a sound principle, and is the foundation of economics. But is it best to repay in kind, in specie? Sure, the money is theoretically unencumbered, so preferable to the lender. But look what we have now - we started from that premise and got fractional reserve banking, now with no reserves, and central banks. There's a human cost to this, and it can be measured in lives lost. There's an alternative : repay the value of the time via ownership in the productive asset. If you lend me 1 btc to buy a car, and I repay you the principle plus something for the time without your capital, which you could have applied to other productive possibilities, then we can both be satisfied if the interest takes the form of a percentage of ownership in the car. The car let's me get to work and be productive ; your ownership means you would get money if I sold it, but you still remain an interested party in my success while I use it. That's more advantageous to the borrower, who is the one in need, while amazingly not taking advantage in an unfair sense of the lender. Economic incentive remains, and externalities are neutralized.