It will be very difficult to replace physical money with btc because in most of the parts of world the digitization is under the way and people are still not able to get access of such advance technology. So i think it will take alot of time and hard work with advancement of technology to get btc as world currency.
Bitcoin cant currently be used reliably to pay for things, except for maybe dinner in some odd-ball locations, primarily because of its volatility. Even places like Expedia or Amazon which accept Bitcoin must, Im quite sure, immediately convert the Bitcoin to a currency.