I believe that the world might end up using a single, digital currency, but not as soon as in a decade or two. Maybe in 50 or so years, or maybe even longer than that. Is that currency going to be Bitcoin? Maybe, but not likely. If anything, Bitcoin will evolve into a store of value, and something else will become a currency for everyday use. However, to answer your question, I expect that crypto and fiat will coexist for a long time before fiat finally goes extinct. It will have to happen eventually, but the process will be slow. At first, cryptos will be something used by a rare few, even though it will be accepted by most merchants and services. Then, people will start using it more, and in the end, fiat will likely be a back-up currency, most likely existing only in bank accounts and spent via the credit cards. We are becoming more and more cashless with each passing year, and the change is inevitable. All that remains to see is how long it will take.