But the question will be "Will that ever happen?" One currency ruling the world will have its disadvantages.

One, some countries already have a lot of bitcoins users or some countries have big bitcoin users while there are countries that don't have much bitcoin users. Basically some people already have tons of this currency while some have very little or none at all. This is an abnormal distribution of wealth. A regular country might suddenly become a dominant country only because they were the first to adopt bitcoin and the nation was able to accumulate the most bitcoins. And one country might start failing just because they were late to the bitcoin party.

Two, there's the digital divide problem. Some countries simply don't have a decent Internet connection. You might be living in a country or in a city where you can get a signal reception everywhere. But there are places in this world that's not uncivilized but the Internet service providers are just so shitty. This limits us from doing transaction. Some people also do not know how to use the computer. Some don't even have a Smartphone. Digital divide has to be solved first before bitcoin can stop depending on fiat currency. Otherwise, fiat will always be there and will always be dominant in some places of the world.

The best case scenario will be that bitcoin and fiat are coexisting and will stay the same until digital divide is solved.