Well, maybe bitcoin can be an alternative to fiat, but i also think that it will not replace. There are so many things to consider in bitcoins, such as unstable prices. It may be very difficult if bitcoin replaces the fiat.
Indeed, bitcoin is an alternative currency and dedicated to online used. But locally, fiat is very useful and I don't think it will ceased to exists in the future. Also in addition to that, not all people are tech savvy enough to used btc and adoption to some maybe slow.