Some people believe Bitcoin will not replace sovereign currencies for non-technical reasons.
I know, and I don't find those arguments very compelling. I think security and performance are the ONLY obstacles to a crypto-currency becoming the world's reserve. I'll explain why below.
Governments are reluctant to give up their ability to control their own money supply. Governments have the trump card of demanding payment of taxes in legal tender which means there will always be some demand for sovereign currencies.
I see this argument a lot and I have to say that people who make it are failing to think things through. The very fact that citizens face taxes and inflation in fiat currencies is in itself motivation for citizens to abandon fiat and move toward BTC. As more and more people do so, the ability for governments to tax drops off dramatically. As government's ability to tax disappears, so too does their ability to enforce tax laws (and all other laws, mind you).
To put it in a nutshell, governments may be reluctant to give up their power, but that's kind of like saying a terminally ill person is reluctant to die.