this would just lead to a socialist society and take us back to the dark ages. Currency value at a very broad level is representative of the living standards in a country. For example, dollar against euro or euro against peso etc. Currency is based on the gold standards. Now, if all countries have a common currency, the living standards are assumed same everywhere, which would mean that a country where infrastructure is poor and resources are scarce would still have the same currency value and they can afford the same salaries and amenities compared to a country where infrastructure and resources are developed. This would mean a disincentive for the country to prosper since it can already afford everything. In turn, countries would start hording on gold to increase the number of currency bills they can publish. This in turn would lead to abolition of the gold standard. This in turn would require a new standard and you would go through the same cycle again where someone would suggest this idea again.

Now, the idea is for each country to have organic growth so that they can strive for better standards of living and in turn higher currency value. The whole idea behind currency is economic incentives to prosper and to maintain standards of living based on growth and infrastructure.

Please understand that currency is just a way of maintaining living standards, it really has no political motivation. It's a good thing we have currencies. What you are suggesting might work in a perfect world we absolute honour and no ambition. But that would be a pretty dead world, won't it ?? !!!