People talk about Bitcoin one day becoming a global currency but something we don't consider is a currency is anything that you can trade goods and services for.
Money (Fiat or cryptocurrency) is only worth what people value it at.

Bitcoin is valued at ~330AUD$ and is used globally.
Bitcoin already is a global currency. It is used around the world and is technically valued higher than any other currency in the modern world.

We're already there, lets keep going.