Bitcoins could never replace fiat currency. Why? because governments wont allow it to, just like US government wouldn't allow Saddam Hussein to denominate Iraqi oil in Euros.
There's always a conflict of interest and every government want their own to be more popular as that means it's more powerful. With bitcoin, there is no government that controls it due to it's decentralized by nature so we are just enjoying it regardless of our races because it's use worldwide and getting popular now. It's popularity somewhat has some limits and that limits is being an alternative currency and not a legal one.
and I think it would be best if it stays this way to avoid powerful greedy people from manipulating it. as it stands, I think there's not going to be any change when it comes to fiat being the global monetary system