Of course, Bitcoin can be a world currency. However, Bitcoin is not valid in all countries. But seeing the popularity of Bitcoin,it will be the world currency for a time.Only the bitcoin has been known in all the countries, which could not make any other coins.Everywhere they are Bitcoin,world Currency Will be.
Bitcoin is decentralized money bro until it makes the bitcon as a legal money. You will find the bitcoin price to reach more than 50k USD in 2020. That time you will see the bitcoin has been adopted by the many countries dude.
So that you itself bitcoin is global currency. If you get into the Japan airport, even now itself you can access the bitcoin ATMs at there and can legally used.
Decentralized does not mean it is illegal and being legal does not mean it is going to be adopted as a currency be the country that has declared it legal. As it is, it would be classified as an asset and just very few people who really understand how it works will use it as a payment tool which we all know is restricted.
But in near future when some reasonable amount of people come to know about bitcoins and adopting it, we can easily transact with bitcoin payments which may be a first step toward global currency. I feel it make take some decades of time.
We should always consider those with limit to technology and internet, as well as those who prefer the traditional way in our equations. Like you said, things are gradually changing with Japan being in the forefront of things, but so far, it is just going to be what it is, and that is a digital currency accepted by SOME.