i wonder: if Bitcoin will become the main coin, what will happen to the money we are using now like us dollar?
Government of any country or state will never accept bitcoin as a main or national currency because of the following reasons.
1) Bitcoin is decentralized having no central authority to control it, government will never allow a currency which is not in their supervision and control.
2) Transactions are anonymous and it will be difficult or impossible for government to check the transaction details of anyone.
3) Taxation issue. It will be difficult to collect taxes from the users over the network.