Unfortunately countries can (and probably will) use bitcoin as an actual currency. They can pay government employees with bitcoin.
Country adopting and explicitly using bitcoin will diminish the ability of the state to collect taxes, but it WON'T eliminate it.
El Salvador is ruled by a dictator. In this specific case the country has no control over its currency, which currently is USD. Bukele loses zero power by switching to BTC, since the switch is from one form of money he has no control over to another form of money he has no control over.