I think the answer to your question is quite easy, first, we all know that the governments around the world are now finding ways to start regulating bitcoin and the entire cryptocurrency market altogether, one of the reason why the government is making this move is so that they can begin taxing bitcoin and cryptocurrencies holders as well.
Taxing bitcoin and cryptocurrencies holders will definitely become another source of income for different governments around the world, but even before this begin to happen, there is currently no place or country where we can say that Bitcoin have become their only means of saving money, and beside, government have other source and avenue through which they are making money from their citizens, example is transportation, government owned schools, oil and gas, real estate export of local materials and import of foreign foods etc.. Government are making money from taxes collected in all of this businesses and more, they can always fund their budget through this.
When the world change with innovation, people as citizens change themselves, business companies and system change too so governments have to be very adaptive with new technological innovations and social trend in order to do their regulations well. When citizens already changed their practice with their monetary flows from traditional financial systems through banks to blockchains and cryptocurrencies, governments have no other choices rather than adapting to this trend, upgrading laws and regulations for better enforcement. At least they must maintain almost similar regulation efficiency like before appearance of Bitcoin and cryptocurrencies.
It's their minimum objective but we know that with innovative blockchains, governments can do their regulations much better and efficient than with traditional accounting and taxing methods and tools.
