Lending has to be against a basket currency of things like commodities or equities that have more stable prices, e.g. a credit representing the S&P500 or a small quantity of the food staples of the nation.  You still run a large risk that your Bitcoins or gold you cash out could be worth much more tomorrow, but they might also be worth much less.

It will probably settle to just having a bank that deals mostly in these equity/commodity notes with a little Gold/Bitcoin/whatever backing, e.g. the Deutsche Bundesbank (with 3000 tonnes of gold), and this will end up the bank people actually trust.

Bitcoin thus serves a role in wealth maintenance, like gold, for when the markets trainwreck themselves by gambling with fiat inappropriately.