It has been foretold that a cashless economy or society will emerge. The US$ and majority or all of the world's known legal currencies will seize to exist. Bitcoin may be decentralized and that the government may not have any or full control over it, but I'm pretty sure they'll find a way to identify who owns BTC and then try to manipulate these people or even worse, force them to give up their holdings. This may be far into the future, but it is the most probable thing I see will happen.
Probably. However, there's a slight possibility that Fiat currencies will continue to exist within the future. Given how difficult it is to stop decentralized cryptocurrencies such as Bitcoin or Ethereum, governments would still support Fiat over crypto. Traditional fiat currencies would simply shift towards a blockchain system (either public or private) for the masses to use. The benefits of this would be greater transparency, eco-friendliness (no need to print paper money which is harmful to the environment), and convenience (by not having to carry cold-hard cash at all).
As such, I believe that a centralized fiat currency has better chances of becoming the world's reserve currency than Bitcoin. Right now, the US Dollar has the lead, but everything could change anytime soon. No matter what fiat currency it'll be, one thing for sure, is that the economy will become fully digitized within the future.
Nonetheless, it's best to name Bitcoin as the People's Currency rather than the World's Reserve Currency. If it remains decentralized, then it'll remain with that title for many years to come. However, if it doesn't, then the chances of becoming the world's reserve currency would be greater than before. Just my opinion
