Here is the detailed explanation from a user’s.

Assuming the whole world starts sharing a common currency whilst maintaining all existing border controls and barriers to trade and the movement of goods, capital and labour, there would be some fairly disastrous effects.

(TL;DR: most countries would exist in a state of disequilibrium, with very high unemployment in some and very high inflation in others, due to asymmetric economic shocks. Exporting firms would find it cheaper to obtain finance and...