Then it wouldn’t be called science fiction
It would be really strange that human attitudes can change in the sense that people should not be enemies to each other - but the problem is not ordinary people who just want to live normally, but those who have the power to control all these people. Is the average Russian eager for war with Ukraine or is the average American interested in imposing sanctions on China or Iran? Of course, this is never the case - corrupt politicians constantly want to keep nations in some sort of quarrel so that they can still enjoy the power that comes from all the chaos they produce.
even though we can't be one nation, we already have the United Nations
there will never be a single currency in this world as cryptocurrency is not just bitcoin
Do you know why the world is more peaceful now? In the past, all the power was consolidated to the king and the king can do anything they wants to. But now in a democratic country, you can not do that so easily. Also, nuclear power gives some balance to the world power and that is why North Korea can threaten the USA but fear to do any military conflict. In the past biggest and more trained army win the battle but now one single nuclear strike could turn the tide no matter what's your army size is. The nuclear weapon is the most dangerous weapon in the human's arsenal and this is the reason we haven't seen any world war till now. World leaders understand mind your own business is good for everybody.