All government backed currencies derive their legitimacy from the government and give power and control back to the governments in return. The system works well if you assume all actors are honest and have the general population's best interests in mind.

History has shown that is hardly the case. More often than not banks, governments and businesses collude to create conditions where inequities keep increasing and those at the bottom continue to serve those at the top. This is enabled by an unlimited currency supply which only gets inflated and the burden is passed on to the common people.

Bitcoin has the power to change this equation with it's property of being a decentralized currency that does not need a trusted party like the government to give it legitimacy. It is power to the people as people choose to believe in verifiability and robustness of the underlying code and network rather than the central bank's word. So yes, As far as the long term effect is concerned, it is definitely a threat to established currencies.