We had a discussion of this idea in the Zidisha member forum some time ago, and most participants indicated they would lend less if they did not have the option of receiving their funds back.
A greater concern is that turning the loans into donations, even though the individual borrowers are expected to repay them, could change the nature of the relationship between lenders and borrowers from one of mutually beneficial partnership, to that of patron and dependent. I don't think this development would be good for borrowers or lenders.
Our current position is, as you say, somewhere between the extremes of for-profit lending and outright donations. Some would call this subsidized provision of capital or "impact investing." As a pioneer in this space, we are still adjusting our positioning as we learn with experience what works best.