The moment all the countries convert to crytocurrency would be the beginning of many amazing things to happen to the world and the financial sector, the process sending and receiving funds worldwide would become very fast and easy, it would lead to growth of businesses and make life easier for citizens of the world.
I agree with this. The currency will come to a new face. The cryptocurrency will lead the world and help everyone to have a better life. Easier and accessible transactions will be done at the same time fastest way. This will help our daily task to be an instant. This thing is not impossible to happen so lets just hold for now.
There is not a single chance that will ever happen but if you want a realistic answer just take a look at the europe. People have been working as a together one country on euro and using it as a single currency for so many countries.
If you do that on a global scale you can have countries with less economical problems, if there was one currency in the whole world than you could have done better with countries like venezuale and turkey and won't have those however countries like greece would cripple everyones economy even if you are doing well. So you are giving at the same time but getting nothing in return aside from headache.