Bitcoin is certainly not a globally accepted currency and it's not currency at all it's just used as store of value. For bitcoin to replace fiat and become globally accepted currency it would require some drastic changes to be made in bitcoin. If there's something which cost you $1 in fiat currency the same thing would cost you more than $1 because you have to add network fee to $1 so obviously you will choose to pay with dollar instead of bitcoin so it's not just instability in the prices of bitcoin which stops it from becoming a worldwide currency but the other factors like transaction fee etc also is a hurdle in bitcoins becoming worldwide currency