But in the far future it's still possible for bitcoin to replace fiat. Imagine worldwide transfer without (or with low) fees. If everyone has their own wallet, I don't think it's hard to adapt with bitcoin 'coz our world is currently full of technology. We will be filled with high technology sooner or later. Well, that's what I believe, at least.
When we rich that digital age, when everything's done on the computer including transactions and stuff. I believe there's a chance that most people will use bitcoin instead of fiat. Thus, eventually making the world not using fiat.