The global currency is the one that facilitates cross-border trade, including investment and international debt markets. Global central banks hold foreign exchange currencies to protect against significant fluctuations in exchange rates, as well as in exchange rates. I think we are a long way from when Bitcoin will be able to do that.
If you look at bitcoin it is probably the best currency to replace any dominating currency for international trade and the only negative aspect you can tell is the high volatility and you cannot expect a currency to have really huge volatility and the traders will not trust anything that changes the valuation in a short period. If we overcome all those negatives then bitcoin could be a good alternative.
Bitcoin is a failure at transaction capacity because the core devs have limited it.
You can buy a $20 meal with the US dollar and not pay any transaction fees.
Bitcoin , you have to pay $20 plus a transaction fee of $5 to $20 dollars, only morons will do that.
US dollar has unlimited transaction capacity without paying a miner's variable transaction fees,
Bitcoin can't replace any fiat currency in it's present pitiful state.