I think in the distant future it is possible that Bitcoin will be used as Alternative currency to buy goods and services but use of Bitcoin as national currency is not going to happens so soon specially in developed world as their economies are based on diversified souces of income and adopting Bitcoin will rise many issues but some under developed might adopt it becuase their economies are so weak and their main source of income is from overseas Remittance like EL-SALVADOR.
I do believe that bitcoin or cryptocurrency will be one of the circulating finances in the world economy but the thought that it would going to replace the fiat's existence I think this one is quite impossible. But for sure if bitcoin will become a legal tender in a certain country the government for sure will going to impose a tax. But for now we all know that bitcoin or cryptocurrency was still struggling to become legalize in many countries and other countries just banned it but still good to know that other countries were open to legalize crypto like El Salvador.