It is still hard for bitcoin to replace fiat because many people don't even have access yet to the basic tools needed to transact using bitcoin. People need gadget, electricity and internet connection and there are so many people who have no access yet to this kind of necessities. In developing countries, some people don't even have electricity so it would be hard for them to transact using it. More possibility is that they don't even know about it so it would be kind of hard for bitcoin to replace fiat during this time.
The only option I can see for bitcoin now is treat it as a store of value along with precious metals and commodities as many are saying its kind of expensive to transact small value transactions so having it as an option for long term investment is a good way of using it. Maybe it cannot replace fiat but it will co-exist with it along with all other cryptocurrencies.