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.
It is a good asset, most of the bitcoin users try to earn or buy bitcoin to they can have profit from saving it over time. It may be a second choice to fiats especially bitcoin is the first digital currency. It will be just used online and to earn profit but it will never replace fiat.