While I do agree for other reasons, the problem you state can be solved by being paid with bitcoin. Some employers (notably, police) do have the option to pay wages in bitcoin. Though these are very few they are still there.
yes it is looking difficult but i think in future when all the people of the world will reject fiat and will start using bitcoin then it will become possible that bitcoin will replace fiat.
There is a possibility but I can say it is very slim. People will always use the currency issued by their government. No matter how good Bitcoin is, people will still believe in the currency that is supported and endorsed by the authority. Even if Bitcoin had been approved and acknowledge as currency, I still doubt that it will replace fiat.