Just turn this around! Its impossible that bitcoin can replace FIAT, which somehow still seems to be the silent dream of many participants.
Says who? It used to be thought impossible to put a man on the moon.
Nothing is ever impossible. Bitcoin still leads and Thus, It is the most accepted Cryptocurrency. Bitcoin may be replaced in the future but I'm not sure It will be anytime soon (Maybe in a decade or two )
If you consider that the block size will increase throughout the years to match demand where the activation of segwit is going to be a first step, then we're already moving forwards.
Another thing is that we also have lightning network that at some point will get implemented to make Bitcoin yet again a bit more future proof.
That's the beauty of an open source project that Bitcoin is. Improvements can be implemented in order to update Bitcoin where needed.