Let's say we have 21M bitcoins and a free-market economy based on them. Everyone has some of those bitcoins and are exchanging them with each other for services and goods. Now, if I am a bank, how do I get more money to pay my lenders? I understand that the things you can buy with your coins grows overtime, but how do you get more money itself?
Banks would need to take control of the 51% hashing power and propose to move decimal places from 8 to 9 (or so). But to understand what I've just said requires that you change your thinking from deflationary bitcoins to inflationary satoshies.