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.