Bitcoin will go mainstream without ever "taking over" from national currencies. That is my prediction. Countries just won't allow their sovereignty to disappear. Bitcoin will be a complementary currency and do many things better than traditional fiat, but it won't replace fiat, not in our lifetime.
Even if Bitcoin is not replacing other currencies, the example is still valid: I lent you 1000 bitcoins in year X and you pay me back 100 each year. Maybe after 5 years the value has gone up by 100x. How can I possibly pay back my debt?
In a XBT-only economy there will not be any 100x gains to be had. Once market saturation is reached (whatever that may be: bitcoin/fiat mix, bitcoin/altcoin mix) then bitcoin value via-a-vis products will be much more stable. Debts will still be taken out, but for production rather than consumption. Interest flows to lenders are offset by cash flow into the economy due to bankruptcies. Only successful borrowers prosper as bailouts end.
The real question is "How can debt work in a fiat system where ZIRP, TBTF, CB printing, bank bailouts, 30x bank leverage, are rampant cancers?"