In the first place, Bitcoin was not designed to be a replacement for the fiat currencies, rather the idea behind it was to address the many anomalies surrounding government issued money and then pointing the way forward for the future of money. In the long run, fiat currencies are likely to get modified but replacement is simply not in the offing, because the governments would never support anything that robs or limit its power, which is one thing Bitcoin has the potential of doing
They may have no other choice
What makes me think so is how governments evolved through history. Emperors, pharaohs, and kings had absolute power in the past claiming that they had been bestowed with it by gods ("all authority comes from God"). Nowadays, we are far away from that in most places of the world, and that seems to have been a natural course of events. In this way, it doesn't particularly matter if they would never support anything which takes financial power from them since if this power is to be taken it will be taken no matter what