If the world uses a single currency, will the economy be more stable?
This is a very interesting question. The sovereign nations issue their own currencies and are taken for granted by many people, but few consider whether this is the economically optimal option.
This is something that can not happen. Maybe you want to express bitcoin will be the currency that people all over the world use. However, you ignore the fact that each country has a different national situation, different historical cultures and developments. It is very difficult to unify.