For a start the relative wealth of nations and their economies would never flourish under a single currency.
We have seen from the Euro that differing economies doing well under a single currency is a pipedream.
You'd have situations where in Uganda, a bottle of water cost 50,000 worldies (my name for the world currency), and in England it would cost 2 million worldies, in line with earnings. You'd have floods of Brits trying to import everything from abroad, essentially killing off all UK manufacturing and resigning the UK to the ****ter, as there would be no jobs.
An attempt to have a single world currency and government would also trigger untold amount of wars as opposition to the batshit idea manifested.
There are people who would rather nuke the entire planet than do what you are saying.
Its complete pie in the sky.
The only way that would ever happen, is if we are invaded and conquered by an alien race.