They could put taxes on it if they could go through the blockchain, but they can't, so doing such thing is impossible for them. Legalizing it will just increase the chances of bitcoin adaptation but it won't change its nature which is being decentralized, and regulating the flow is just a dream for them, bitcoin is not fiat, it is a digital currency that could run on its own without anyone behind its back, think of it as a super AI network.
Bitcoin will remain a digital currency and not the same as fiat. So, it's not gonna happen they will convert into cryptocurrency and leave their own local currency. Each country has its own revenue or era and that what base on local currency value. There is a chance that it will alternate currency to fiat through the internet payment system.