Eventually, one of the blockchains will become truly global. But before it becomes widely distributed, there needs to be important infrastructure changes:
It needs to be trivially easy to exchange a cryptocurrency for goods and services. There are only a tiny percent of businesses accepting cryptocurrency, and those that do sell immediately from crypto into their fiat currency of choice. Cryptocurrency doesn’t do me much good if most of the stores I use don’t accept it.
It needs to be trivially easy to exchange my fiat currency for cryptocurrency. In order to exchange Bitcoin of Ethereum, I need to sign up for an approved exchange to purchase ETH or purchase from someone in person. For anything other than Bitcoin or Ethereum, it’s an even more difficult process. Cryptocurrency doesn’t do me much good if I can’t easily exchange dollars for cryptocurrency.
It needs to be trivially easy to secure my cryptocurrency while being accessible. Most of the people who have significant holdings in BTC or ETH are afraid their going to get hacked or lose their private keys and passwords. There are not many services that make this balancing act easy. Cryptocurrency doesn’t do me much good if I can’t be confident I can maintain my holdings.
These innovations will take time to develop, which means that the first truly global currency will take some time to develop. Assuming Bitcoin doesn’t get taken over by another currency, it could very well make the jump to become a major currency.