75.9% ( 15,947,975 bitcoin) of the bitcoin is already mined so to circulate bitcoin all over the world there is only few bitcoin because many bitcoin user are holding bitcoin and waiting for price hike. So what would happen if the continue holding and we are left with only few bitcoin to circulate. So can it act as global currency or we need some modification.
Even there are only few thousands bitcoin in circulation it can easily work as global currency as even 1 satoshi is transfarable however if supply get limited than price will be high for sure which will make bitcoin mining even more profitable. I don't know how you have predicted that majority of bitcoins that were mined are not circulating. 