As a side asset, to keep along with fiat, bitcoin is definitely the best. However, at its current condition, it can, in no way, in the future, try to even remotely replace fiat. I mean, come on...bitcoin is extremely volatile, how can its popularity go beyond something that has been used for centuries traditionally, when people are not even trying to trust and use it.
Moreover, not many people are familiar with the concept and not educated enough to use it, and lack of adoption makes bitcoin's value unstable, and this cycle continues. Thus most governments aren't taking the risk to try and implement it mainstream, as if they use it as official currency, their economy's value might zero down in days along with bitcoin's if and when it falls.
Well, the fact that fiat has been used traditionally and long enough is not even the issue for me because there is a general saying that the only thing that is constant is change. Therefore, a kind of change or policy can come up now that might change so many things including the use of fiat nor bitcoin. The issue for me is that bitcoin is controlled and before a government would throw itself fully and jettison the use of fiat, it must have a way of controlling nor checking the use of bitcoin and its activities otherwise, politicians would loot uncontrollable.