If you're expecting for massive adoption and to have bitcoin as the national currency, I could say that we're still far from it. Bitcoin could be a mode of payment but merchants still don't directly accept it so fiat is still necessary. Fiat would always be involved in every local transaction despite the existence of bitcoin because it still needs further development and total adoption due to its volatility.
As a national currency, I am sure that Bitcoin will not be used in any state. The national currency is always tightly tied to the state of the state economy. The price of bitcoin cannot be tied to the conditions of economic development of any state. Bitcoin can go in society along with the national money of states, but can never replace it with itself
I mostly agree with you
With that said, though, we must remember that there are countries (like a dozen or so) which don't have their own currency, and are using a foreign currency instead. It is mostly the US dollar throughout the world but the Russian ruble is also used in certain so-called unrecognized states in lieu of a full-fledged national currency. In this way, it is quite possible that a particular country could adopt Bitcoin as its legal tender on equal terms with some other currency, especially in the cases mentioned
As for the volatility of the price of bitcoin and other cryptocurrencies, it will always be very high, as long as the cryptocurrency remains decentralized. No other factors will significantly affect this
I agree with that too
But volatility is most certainly to stay not because of a decentralized nature but rather due to the speculative essence of cryptocurrencies. After all, gold is also decentralized but its value is a paragon of stability. If you see its price change, remember that it is not gold's value per se that changes but rather the value of the currency it is priced in. Gold itself is rock-solid (despite being a soft metal on its own)