The limited number of bitcoins of 21 million is not related to its possibility or impossibility to use it as a world currency. The impossibility of such use of bitcoin is associated with its high volatility. It is the instability of bitcoin that prevents its use as a world currency. If the price of bitcoin is constantly changing, the economy of the country will not be able to work normally. Therefore, bitcoin as money can be used only along with the usual national money of the state.
You have a point to an extent.
Crypto is a very new type of currency.
15yrs is nothing compared to few thousands of years that paper money is being used, or even gold.
So BTC will stabilize in the future, maybe in 10, 20 or 50 years but that won't be enough for it to become a single world currency.
We don't have yet the single world currency because it would need one central bank that controls it. Every nation has it's own so it can control the economy and inflation with it.
In EU they have euro but every country has it's own fiscal policy and that's the reason that euro isn't doing very well.
Who would control, and how, the bank of
BTC?
And why would we even allow that?
You said it, the BTC is probably going to live alongside national money and that's it.
But it can, and will be, used by the whole world because there is not obstacle for that.