Bitcoin will not be able replace fiat for now because of several factors. Among them are unstable bitcoin exchange rates and international laws protecting bitcoins dont' yet.
The factors which you provided is not any main problem which will hurdle bitcoin to replace fiat because the unstable rates on exchanges are due to low number of people on bitcoin when the users of bitcoin will increase then a stability will come regularly and as the users increase the international law will come in favor of bitcoin.
But I guess it is not about the number of the people using it but the only difference that differentiates these both is that the BTC is a decentralized currency. No country will trust it till the end day when it comes to a national issue. We need to make it clear that the fiat is backed by a country or a state with delivering its strength to the others and there are many national issues related to a currency. Second the store of value is a real issue.