the bitcoin, ethereum or other cryptos has to be world currency because people must have an alternative to Central Banks Fiats Money,,,,, they are just Printing and distributing to other banks(bailing them out),,,,,
You can't do this forever,,,,,someday people will know your(central bank's) misdeeds and they will adopt other Trusted Means(bitcoin or other cryptos) to avoid their Fiat Currency...........
Cryptos empowers people more than anyting else................