I am convinced that there will be no one global currency. Each cryptocurrency has its own unique properties and scope. If bitcoin is a "digital gold", then Monero focuses on increased anonymity.
The good thing is we do not need to have only one currency, there is going to be a dominant currency, like the dollar right now or the British pound before that, right now the dominant cryptocurrency is bitcoin, and it is going to be the most dominant cryptocurrency for the foreseeable future but that may change, after all if satoshi nakamoto created bitcoin what it is stopping another satoshi nakamoto out there to create something even better.
We know that developed countries are using the Bitcoin even in public places as payment and somewhere but highly using between companies and other transactions between countries on money related basis. The future is very bright as we know that Bitcoin started from 1 dollar and now even its lowest value is 7k dollar so this indicates its success and future growing with time.