Do not expect for any cryptocurrency to replace the fiat currency. Even bitcoin cannot replace fiat currency and will remain only a currency online. In the future if all sectors of the society will accept bitcoin as mode of payment it will still not replace fiat currency. Its value will still be determined by the fiat currency and as long as it is not independent from fiat currency it can never be the universal legal tender. Bitcoin and other digital currencies are weak since it is dependent on internet and is very prone to hacks and viruses or technical errors that may be a huge problem in the future, thus the central banks all over the world could not allow such kind of risky currency to serve the world as an official legal tender.