Bitcoin can only be viewed as an electronic currency, widely used on the internet worldwide, but to replace it with a solid currency, this is not possible, the reason being:
1. Bitcoin is an anonymous coin, unable to determine who sent the recipient, leaving the world out of control of the coin!
2. Bitcoin can not impose taxes, which makes it difficult to develop a nation's infrastructure
3. Bitcoin is not safe. Frequent congestion occurs when financial markets are volatile
4. Bitcoin is a perfect place for criminals to conduct transactions, causing global turmoil due to its anonymous ability (ex: terrorism, drug trafficking and weapons, extortion such as viruses. WannaCry ...)
If Bitcoin can overcome the following factors, I think Bitcoin can replace the Dollar, but that's a distant thing.
1. And then? You received the coin. That's it. If you mysteriously received some coins, then thank you to whoever sent it to you. It's pretty obvious that the sender was the guy you were having a deal with since it's pretty unlikely that someone will mistakenly send money to your address.
2. Tax will be paid just like the same way. You buy a food, tax is already there. Before your salary goes to you, the tax has already been imposed.
3. If bitcoin replaces the dollar, the prices will depend on bitcoins and not dollar. Volatility isn't a problem anymore.
4. WannaCry? Really? It's just a recent thing and now it's a reason why bitcoin cannot replace dollar.
Fiat is even more anonymous since there's no way you'll know who held the paper before you. In bitcoins, all the transaction history is out there.