I think bitcoin will not replace fiat because fiat is our foundation of money, and if bitcoin will be more popular than fiat then it is good, and I'm sure it will happen. But I don't think it will be gome completely because it is still important for those who doesn't have access in internet.
It's quite impossible that btc will replace dollar or any fiats for now,it will only be wudely used through internet for easily transactions but in most common transactions fiat as well as bank is still needed.