Even if Government don't support bitcoin there is nothing to worry as bitcoin is decentralized currency even Government cannot control it so bitcoin will never disappear.
Bitcoin can be used now in different countries even without the support of government so nothing to worry even the government will not support bitcoin.
Also we can able to save taxes here although it will give economical effect in one country.
I don't think that fiat will ever disappear in future as it holds support from the Government but same confidence we can't put on bitcoin as if it falls to extreme level then people may stop using it.