A currency for the future does not mean that it has to replace the paper currency or a national currency as such, it will be an alternative for international trade and is the cheapest form of transaction even if you are dealing with millions of dollars worth of trade or be it a billion.
Indeed, I don't expect Bitcoin to replace national currencies. As long as states exist, they'll continue using currency as a means for economic stability and control. I do expect central banks to eventually start holding Bitcoin reserves as a hedge against currency risks, though.