Have always wonder which currency would the world accept if only one currency was to be used. Maybe the dollar?, don't think so, The British won't tolerate that.
Cryptocurrency?. Maybe. Until the advent of cryptocurrency, I never thought that would be possible but it seems possible.
The world is not too far from a unified currency.
Ask why we need to use one currency instead of asking what currency can be used as universal only currency?
There is no reason why we need to stick with only one currency even though it is bitcoin because its literally not possible to make every transactions in this world with only bitcoin because we know how much we need to pay for a transaction when the network clogs at 200K transaction so imagine how much the fee will be when we stuck with 1 million transactions.