Bitcoin is already a global currency, albeit of limited usage.
The real question is "will it become a global reserve money?"
... or remain a global transaction commodity, like gold.
I doubt if it is a global currency now or not. It is not supported by anything such as gold that all of currencies do have it and it is needed for all currencies.
Most of the nowdays currencies are not supported by gold or anything, although most of the central banks has a piles of gold in the trezor and has also some dollars and euros to back up their currency on the forex market. The quantity of these are not enough for the value of the whole country's currency, it's only held in order to do some interventions on forex market if needed to maintain the value of their own currency.
So to sum it up in a few words, bitcoin is just like real world currencies, there's no need to be backed up with gold or similar. As long as people trust bitcoin, it won't fail.
If you check the news, some giant banks are planning to use the blockchain technology, so the direction is OK at least.