I agree, currently bitcoin already used a lot of people and all States are satisfied with the system of the bitcoin transactions, compared to conventional bank then bitcoin better.
If the term global is used to determine the usage of bitcoin across most of the countries, then bitcoin is already a global currency. Only thing it needs is legal adoption, which is quite not possible to be got in a short time period.
Global currency does not mean used by fraction of the fraction people from some countries. Bitcoin first needs to prove itself as scalable to other currencies. look at the uncleared transaction on blockchain. We want bitcoin to scale up it to such level that nobody should doubt about it. Right now that is not the case. There are lot of doubts about the voilatile nature and features such as microtransactions missing.
Lets hope bitcoin will tackle every challenge successfully
This is difference of degree, not of kind
In other words, it could be said that it is a matter of sheer adoption and expansion of Bitcoin user base. Regarding the need of proving itself as scalable to other currencies, I don't think, first, that Bitcoin needs to actually prove anything to anyone, and, second, what other currencies are you talking about? If you mean digital fiat, it is not scalable on its own, you need banks and payment processors like Visa and cronies to do the dirty job. Cash is not scalable altogether. Scalability is an issue, indeed, but it is in fact a minor technical issue, which will get resolved "sooner rather than later"