indeed on the scale you provide, all that is true. Bitcoin cannot become a global currency due to large fluctuations, privacy, anonymous nature. so it is very difficult to entrust this to a country. and the weakest for currencies is, basic decentralized. this is the main reason that bitcoin does not qualify as a currency. and will continue to be a controversy in the future.
How is it possible that you believe that decentralization is a weakness for bitcoin when that is what it has allowed bitcoin to still be here 10 years after its creation? Satoshi knew very well that if he created a coin that was centralized then governments could destroy it as soon as they decided to do so as they have done with many services that do not abide by their rules, satoshi made bitcoin decentralized so no government, bank or any institution could kill it.
It is already a global currency because Bitcoin spreads the world now and majority of the world are accepting Bitcoin
already or even if they don't accept at least they
are not clearing this as illegal currency.there many countries now in which treating Bitcoin as their legal tender meaning
we already have this as transaction paying currency
.And being decentralized is not a bad approach because yeah there are some government that don't wanna put this in
to their economy but in future i know,they will understand what
this is all about.