it is very difficult for bitcoin to become a global currency. because there are still many people who do not know bitcoin and there are still many countries that prohibit the existence of bitcoin. because they fear that the development of bitcoin can not be overcome. but you do not have to worry though some countries do not allow it but in some countries, many have received and legalized bitcoin. and I'm sure bitcoin will go on and on all over the world.
Yes it is difficult but not impossible, i think it will take a lot of time, but still it is possible and it is going to happen, because bitcoin is slowly but regularly increasing its popularity, i am hopeful that a time will come when bitcoin will be too much popular and about all the people of the world will be using bitcoin.
It will take some time but things are going to accelerate when the economies of the world begin to show problems, right now the economy is still more or less working but we are seeing the first signs this is going to change, the trade wars between the US and the rest of the world have began and history shows things do not end well when we have a trade war.