It matters how far crypto is allowed to reign in a country to see what it would do to that country. If Cryptocurrencies are regulated, of course it's not going to be allowed to disturb anything that's going on. But if it is given free will, then Bitcoin and the other cryptos will SLOWLY (most likely) take over a countries finances as people switch to it as they'd have no reason to.
This government would probably lose economic control of their country, as they can't issue currency, raise interest rates -- etc.
It would totally disturb a country's economy.
That is a valid point! Yes, in as much as the good sides are there, the bad sides are also there. This is a decentralized currency and if everyone decides to start making use of it, abandoning the fiat system, regulation could be very hard. The reason why regulation is still a bit easy in some places now is because one way or the other, we can still convert to fiat.
In a system where the P2P transaction is solid enough, then we should see so many people trying to evade tax, but I still feel in a way this can still be controlled and after all, in some places, we have property tax and if people are doing well, that should really help.