With today's technology, it is not impossible that Bitcoin will take its reign to be the major fiat currency in most country. What do you think?
I think as of now, because not everyone has access to Bitcoin and bitcoin transactions, we will not see it take over fiat for a while. With bitcoin, every person will need some form of device that allows for transacting money. A lot of the population is in rural areas, are poor, and may not even have access to things like electronic devices. For now, fiat offers a simple, well accepted currency based on a country. If we can get to the point where everyone has an electronic device or that every government provides it, then only will I see it possible for bitcoin to take over.
i don't think OP had replacing fiat in mind.
in any case i do believe that bitcoin can become a big currency and something that is used by many pretty soon as payments. specially in countries like Japan which have adopted bitcoin and removed taxes on bitcoin payments. that is more than enough incentive for anyone to start using bitcoin instead of fiat (like credit cards for example) and pay even less taxes. and shops are already accepting bitcoin over in Japan.