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.

Correct! As Bitcoin is divisible by design, there's plenty of coins available for all the inhabitants of the world. The smallest unit of Bitcoin is a satoshi, which is a very diminutive form of a whole Bitcoin. Therefore, as I said previously, you can calculate how many Bitcoins would be available by just dividing 1 satoshi into the total supply of 21 million. This in effect gives you a result of a quadruple amount of Bitcoins in existence. So, even if Bitcoin's value rises to astronomical levels, it can be divided into smaller amounts for anyone to enjoy.

And, there's always the choice of altcoins which opens up the level of accessibility from everyday people into cryptocurrencies. In the end, there will be plenty amount of Bitcoins for everyone, but not everyone would be able to use the digital currency, as it is not scalable yet. But, once it does, Bitcoin will become a truly worldwide phenomenon.

Bitcoin to me is a digital currency which can be used as store of value , and on the 21mln total supply and if it can be use by whole world , yes it can cos people buy and sell Bitcoin everyday and more demand makes it grow in value which will makes people to only own Satoshi in future but those that own a Bitcoin then will be in money and more people will be looking to have it while it value will keep increasing .

I like to think about 1 satoshi value as 1 cent in the future. In that way we have 21*10^6 bitcoins, each comprising of 10^8 satoshis (=cents) which makes 21*10^14 cents, same as 21*10^12 dollars. Obviously that amount of dollars can be used by the whole population.

Satoshi Nakamoto was a pure genius since he not only created a digital currency with limited supply, but also with the ability to expand itself into many units if it were to be necessary. As such, Bitcoin can be used for nearly anything we could think of since there will be plenty of coins for everyone to enjoy.

No matter if Bitcoin's value would be too expensive in the future, it can be divided into fractions of a whole Bitcoin, to allow anyone to use it. This is a huge benefit if compared against fiat currency, as different divisions of the cryptocurrency could further expand its use cases such as micropayments, colored coins, and more.

Therefore, I think that as Bitcoin fees become cheaper with new scalability upgrades like SegWit and Lightning Network, people can transact with even 1 satoshi in the future. So, even holding a single Bitcoin could give you 10,000,000 satoshi (that's ten million satoshis) which can be worth a lot of money by then.

You forgot about something. 

There are currently millions of Satashi's coins that have never been touched and then you have loads of coins that were send to burn addresses. There are also people who has lost their paper wallets or hard drives and memory sticks with bitcoins on it.

At the rate that this is happening the total amount will be significantly less and we would come to a point where a decision will have to be made to divide a Satoshi in smaller units. 

The system of financial trust based on bitcoin will break down regional barriers so that all human beings can use safe, fast and reliable currencies. Hayek said that money is the greatest freedom that man has ever invented

Bitcoin has the cap of 21 million, but also it is divisible to the 8th decimal place. Thus, there will be no problem that bitcoin can serve the whole planet, given that its price is sufficiently high. By the way, this is one of the reasons I think the price will be high eventually

Bitcoin supply is 21millions and have 100millions per bitcoin. Its mean bitcoin supply is 100millions*21millions because right now people trade bitcoin on sats volume.
On regulation i think its hard to make bitcoin as currency but as mathematical calculation, that could be happen