These days nothing is impossible, and in my opinion, Bitcoin may come to that level of been the reserve of many countries but may not be the reserve of the entire world. Take for instance, Venezuela, many people there are converting their money to crypto because of high inflation and very week currency that has no purchasing power.
In addition, if the trend continues this way, ie where citizens of countries will loose confidence in their currency, it could be rigth to say that digital currency might take the strong hold (reserves) than national fiats. Many nations has made the US dallars as a reserve, but what if Dallar falls? I think that the coming of crypto and the blockchain might be the next reserves, probably in the future.