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the obvious obstacle is recognition from the state, not all recognize and can accept bitcoin and this takes time and cannot be easily resolved quickly.
It must also be remembered that banking is still very much needed because it accommodates manual transactions, and traditional transactions are still very much felt and indeed cannot be avoided and will bitcoin be able to do that later?
It depends on what are we talking about: if about replacing all existing currencies and becoming the only one currency in the world then it has no sense. Bitcoin's idea is to be an alternative, free, decentralized and anonymous. It is impossible to imagine it will be any how mandatory.
When we talk about world currency I mean first being widely accepted for payment, so widely that mostly in every small shop around you would have such an option; and second to be considered as an anchor currency not less popular than some like euro or USD.
I don't think this has to be, that is, for Bitcoin to be taken as a worldwide independent currency. I might not know but I could guess that the global Government might not come to this point ever where all accept Bitcoin as a legal tender except this is not what is been assumed here. As far as I am concerned, Bitcoin is already a global currency because those that have seen it as such have been accepting it for different things even some fiat currencies of some countries are not accepted in other parts of the globe today.
Bitcoin's nature gives an opportunity to use it as you want whenever you want. But there are not many places where payments in bitcoin are accepted at the moment. Right, it will hardly be totally accepted everywhere in the nearest future, but it can be accepted in majority of countries.