it is not possible for fiat money to disappear. the world cannot funtion only using virtual currency. and most of the people do not trust on virtual currency. and it will also be difficult to exchange bitcoins for day to day purposes. it can be helpful in many ways but due to certain boundation virtual currency cannot be only currency used
How will it be difficult to do so in the future, its easy as hell right now? it will be even easier.
In the future (not so distant) there will be physical cash, so the idea of paying with smartphones will be widely accepted. Once that idea is ingrained within people's minds, the idea of using Bitcoin instead of some goverment issued e-currency scam will be a natural one.