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Why is the fractional-reserve banking not a Ponzi scheme? 
It is possible to pay down debt (including interest) without issuing new debt money to pay for it. I think this is the heart of your question. Let me present a highly contrived example in which society has four people and one bank. Here is a bank with $100 in initial deposits. Total money supply in this society is $100. (We assume there is no currency circulating, since you're interested in debt money.) This bank lends out $90 to Bob at 1 year maturity and 10% APR. Bob spends this $90 with Charlie to buy raw materials. Charlie deposits $90 in the bank. The money supply just grew from $100 to $190. Bob does something with the raw materials and adds some kind of value, eventually selling the finished goods for $110. In our little silly economy, the only people who have money are Adam and Charlie, so we must assume that between the two of them they buy $110 worth of goods from Bob. Let's say Adam buys $60 and Charlie buys $50 -- the actual amounts don't matter. Bob deposits this money at the bank. Still $190 of money supply. At the end of 1 year, Bob instructs the bank to transfer payment from his deposit account to his loan account. The bank wipes clean his debt and the money remaining in Bob's account represents his return. Who is this David guy? He's the owner of the bank. He grosses $9 in interest from the loan to Bob, and he pays $5 to Adam as interest on Adam's deposit. The remaining $4 is the profit to the bank's owner. Money supply decreased from $190 to $100 after Bob pays off his loan. I realized after writing this, the one thing I left out is, "where does Adam get $100 to start with?" Presumably Adam starts off with some kind of currency, either fiat money or commodity money. (IOW, debt money can't be created out of nothing, it has to be expanded on top of some kind of currency.)