literally bitcoin can never be accepted as a global currency... the bitcoin is a currency and not real money.. the problem with this is bitcoin does not allow for fractional reserve banking..
i think bitcoin already become global currecny,in my definition global currecny is currency that accepted in so many contries,and now we all already known that bitcoin is accepted in so many countries,bitcoin not must be real money to be global currency,and undecentralized not make bitcoin not accepted,it just matter of time.