I hate to say this, but Bitcoin is too small overall in order for it to be the currency of -whatever-. And I don't mean small as in user base, I mean the divisions are too small.
As it stands, if every satoshi was worth $1, the market cap would be ~2.1 trillion, if I remember correctly, and thus would be too small to replace the current monetary system. Hypothetically it could, but there would be too many outstanding debts, or anything really, in order for it to be able to fully integrate itself into society.
I can see it becoming a global currency since we see a collapse of the current economic system, but that's only because most other money won't mean much anymore.
It is certainly global if you mean it can be transferred anywhere with ease, however.
That's where altcoins are the turning point in relation to that limitation in bitcoin.
Actually I sincerely don't think bitcoin will be the most used digital coin, since there are better platforms out there, better transaction speeds, etc.