Bitcoin or any cryptocurrency or even currency will never be used as a reserve currency because no country would want their reserves to lose or gain value now and then as that will make things unstable and unpredictable. What if a country holds 1 million Bitcoins as a reserve and the price of Bitcoin drops significantly? We know that can happen because Bitcoin is volatile and it moves in a cycle which means that it goes up and also comes down after some time.

So, even if Bitcoin becomes an accepted currency all over the world, it's never going to be used as a reserve. People can use it to store their wealth, it can even be used to pay for goods and services, but as a country reserve, it can affect the economy of a country which isn't a government would want or accept.