What if people convince governments of their nations to make Bitcoin as a legal currency or what if we agree to give tax on our bitcoin income? Will government then make Bitcoin a legal currency.
It depends on what you mean by the words "legal currency". Governments can only allow the circulation of decentralized cryptocurrency in the state, but they themselves will not actively support it. In general, the government is not interested in the circulation of decentralized cryptocurrency. Replacing the national currency with a cryptocurrency, the government will not under any circumstances. This will threaten the normal development of the states economy.