There are numerous reasons which are big hurdle in BITCOIN to become a global currency is the decentralize because government unable to control it.Government experienced many cases that BITCOIN is used in money laundering and illegal funding for illegal activities.
Bitcoin is fruitful for stable economy if government find the way to control and track the transactions,Because transactions will generate revenue in form of tax that revenue can be used for further development and infrastructure of the country.
Unless the USA is the biggest economy and its currency has the status of an international one, there is a very little chance for BTC to replace it. As well, the fiat currencies are way more often used for money laundering and illegal activities. Especially in a physical form.
As well, if it not controlled, country cannot manipulate its own economy in such easy way as simply printing more fiat currency out of thin air, but would need to make serious and unpopular decisions instead. This is why until now, some countries of the Eurozone - like Spain and Greece are not fully recovered from the last economic crisis.
In general, BTC transactions can be tracked, and with current FATF guidelines and direction of new legislation - the general goal is that like with wire bank transfers both sending and receiving party will be fully identifiable.