In short term, I can not see the possibility for the world to be able to adopt and use bitcoin as official currency, because government wants to control something they can control, however, there is no central control on bitcoin due to its decentralized features.
If bitcoin is used as fiat one day, the downside would be all the people will go mining for the "money", because bitcoin can be generate through mining and large amount of bitcoin will held in small portion of people due to early mining.