The problem isn't that there isn't a common-sense distinction between the extremes; it's attempting to define the crossover point (presumably for the purposes of a "bright line" rule in law/policy/norms). Defining how much interest should qualify as usury is a classic "paradox of the heap" [0]; which may be a reason by some faith traditions simply forbade lending at interest altogether.
An interesting alternative to consider, is money that intentionally depreciates value at a fixed rate ("demurrage" [1]). In theory, this would incentivize lending at zero interest (assuming relative price stability for real goods, which is easier said than done).