I think bitcoin will become a global currency only when we make aware everyone from this currency, and in our business we accept only bitcoin and not any other currency .
Awareness isn't quite a big deal. Actually with the current of numbers of bitcoin users it's enough to become this crypto a global currency. But still it isn't considered as a global currency? Because in most of the cases, bitcoin used to get profits. Yes, an investment and that isn't the role of a global currency.
Due to the current low adoption rate, there are still long way to go before the price will be stable. As Bitcoin is improving in its features and usability, the advantages of it is showing up gradually and ppl will realize it.
On the other side, USD dollar will show its disadvantages of being centralized currency, such as inflation, transfer fee, counterfeited notes, space to carry etc.