Nothing is ever impossible especially when it comes to such things as regarding the world and a currency. Bitcoin is as a crypto currency has come a very long way over time and is still going further and father than we ever imagined. And it is still very much possible for Bitcoin to evolve overtime and become central world currency.
Isn't the bold one conflicts the decentralization of Bitcoin? Though I admire your positivity but not all things are possible and I don't think Bitcoin will become the main currency of the future. I believe it is set as an alternative currency due to the systematic way of the world being centralized.
Especially when it is most accepted and used in world power countries. Countries that that are major drivers of the world economy. Countries that are solely dependent on such countries will have no option than to accept the Bitcoin as a means of exchange.
As much as I like your thought to happen, but reality states otherwise. Bitcoin would not be the only option for countries that solely depend on other countries for means of exchange, one reason is the other countries has its own currency, and each country has their own currency so that is many options, and the country's own currency, that is another option. So you see, there are many options for a country to choose as a means of exchange, besides being in a centralized system, it is more likely that the one issued by a government would be chosen as a means of exchange and Bitcoin will remain an alternative choice.