Well it is true that Bitcoin can be use for spending anywhere in the world but it needs more improvement so that it could equal the US Dollar in which it can be use instantly around the world unlike Bitcoin that not all establishments are accepting it and the need for internet is vital on every transaction.
Well, it cannot be equal to USD in the logical sense because USD is fiat and centralized. Sure, the idea of seeing it being used all over the world and being a currency more preferred by a lot of citizens of each country could be possible, but we also have to understand that each government will still want to push their fiat and find a way to keep doing that even if it is on the blockchain to at least still make fiat stay relevant, so, i still cannot capture my head on the possibilities of it working out.
Ths is one of the goals of bitcoin, as to provide more broad and satisfying services and products to its client/customer. I believe that this is possible in the future.
Yeah, a time will come when bitcoin will be easily used for payment purposes, and then people will tend to realize how easy things are meant to be without anyone trying to be in control and in this case the government. As a unified currency even though it sounds cool to the ears, I am pretty sure it is something that the government may not really want to allow, but over time, as adoption keeps increasing, real life usage keeps increasing, who can tell?