In all countries, the national currency is supported by the central bank. He follows his course and, if necessary, withdraws some of the national money from circulation or adds them to turnover, and also buys a foreign exchange currency or sells it. This is supported by a fixed rate of national money. The dollar in us is very special in the world and therefore here the example with the dollar is shown unsuccessfully.
Decentralized crypto currency is not supported by anyone in order to stabilize its exchange rate and therefore as extremely unstable by its nature can not be used as the main currency of a particular state or group of states.
A central bank supporting their national currency is not really a great insurance, many currencies have crashed and disappear, many currencies have been losing value despite the fact that a central bank is supposed to control the emission rate of the currency, so a central bank is not really something that solid, and I will not trust any central bank over bitcoin.