People have too diverse views on poliitics and other human factors, for them to adopt a single currency world wide.
If one is preparing for war, one does not pick up the sling or the spear when the machine gun is loaded and ready.
Bitcoin has already made all other forms of money obsolete. Most of the world simply has not yet figured it out. Rest assured, they will. Sooner rather than later.
Wait ...Bitcoin has not yet solved the offline transaction problem which is required for no internet zone. Cash still serves a good purpose in this regard.
Yeah, all the billionaires in the Gobi desert have no use of magic internet money.