The Argentinean Peso and the Venezuelan bolivar have changed their value a lot, yet they're still currencies.
In 10 years or so, one Argentinean Peso went from being worth $1 to being worth one cent.
And the situation in Venezuela is even worse.
So, I think Bitcoin is pretty stable compared to that.
Bitcoin is a lot better than Fiat currencies because it can be used across borders and gives you complete control of your money. It has done well even with the decaying state of the global economy. If Bitcoin has reached this far, then it can do anything. I'd imagine people using it at a large scale worldwide pretty soon.
The pioneer cryptocurrency is not a "world currency", simply because it isn't officially recognized by mainstream governments as such. But technically, it is. After all, anyone can use it anywhere as long as there's access to the Internet. This is only the beginning of the "Bitcoin Revolution" that will change our world for the better. Who knows if someday BTC beats Fiat currencies for good? Just my thoughts

It would still need some governments from major economies to foster practical regulation and accept Bitcoin for tax payments. They can still pass regulation that makes it overly complicated to use Bitcoin for various purposes. Especially when it comes to smaller scale transactions or micro transactions, they could force people to document everything from the origin to realized gains and losses etc. In the end it is a race that governments can't win, but they can delay the adoption process. At the same time they accelerate it by making a certain spectrum of transactions more feasible.
I don't know in how far technological questions like those about quantum computing and its impact on Bitcoin first need to be addressed and answered in more detail. If a cryptocurrency serves as the world currency, it should at least be clear that it can't be attacked by hidden innovation. What if one government is researching technology that nobody else knows about and it could attack a multi trillion dollar network? Could that result in a serious doomsday?