I don't think that the government will use cryptocurrencies as cryptocurrencies challenge the centralized aspect of the fiat money system. Most major countries have stable governments and a stable economy, which means that the fiat can be trusted to some extent. The government still need to enact monetary policy; they are incentivized to print money to keep a steady inflation to promote spending and adjust interest rates to promote saving money or borrowing money depending on how strong the economy is doing. To create trustless cryptocurrency, the government has to give up these power that are essential to managing the modern economies. I only think that unstable governments will use cryptocurrencies to introduce a stable medium of exchange and gain back trust.