I guess in that way we can use it but there will be one single time when its actual market cap will do reach to the maximum threshold thus leaving us no choice but halting it there. I mean you can cost 1 Satoshi 100K USD as we dont have any further units or breakdown of the bitcoin and thus it will become non trading. I mean even if it becomes worth 10 bucks then what about the smaller things that cost below that, how will you purchase them anyway? That kind of questions might arise in the future.

I think bitcoin wasn't invented to be used as "money by the whole world". And also, bitcoin can be parted to satoshis. But nevertheless, it wasn't meant to be used just like fiat. It is a digital currency which value isn't affected nor controlled by a central entity. And if bitcoins total supply will increase, its value will decrease.

bitcoin is a digital currency and it can be used for the whole world the only problem it faces is that it is not yet accepted by all 

well I cannot say at the moment may be maybe not but one thing is clear it will solve the worlds biggest problem if the whole world accepts it

Of course because 21 million is too small compared to people in the world and i think 21 million of supply will becomre more high price because of the supply and demand i am sure bidding will increase more because of the people wants to get a lot of money.

bitcoins 21m cap is the same as saying there are only 175,000 tonnes of gold

at code level there are not 21million coins going to be produced. its actually measured in satoshi's at code level. and thus there are 2.1quadrillion sharable units of measure(analogy: gold ounces)

but the main point is. bitcoins whole ethos falls flat if bitcoin took over the world and became the only world currency. bitcoins ethos and purpose is/was suppose to be a open barrierless choice away from FIAT. so having a choice and options is good.. but being forced to only use one currency is bad. so dont expect everyone to store their entire life savings in bitcoin.

but yea if you imagine 1 billion(12.5%) of the world using bitcoin partially invested as part of a portfolio then each person can have a noticable amount of satoshi's in their control

why do you think that? did you give it any thought or just a guess?
there are 2,100,000,000,000,000 units currently available to bitcoin. and 1 unit of it is called 1 satoshi which is worth $0.000083 even if bitcoin reaches $1 million, 1 satoshi will be worth $0.01 which is will be the same as smallest unit of USD. so why do you think there isn't enough of it available?

I think bitcoin can not be used for the whole world, because Bitcoin around the world is almost reaching its limit. To date, there are already 16.8 million Bitcoin successfully mined by miners. The depleted digital currency stocks are running low. Because it is only 20% of the total 21 million Bitcoin pegged by Satoshi Nakamato, the creator.

Bitcoin is number one digital crypto currency in the world but there are some countries they are banned bitcoin if you have 1 billion bitcoin but this coins are not accepting by your government then it will not banifited for you.But the situation is changing many countries governments are accepting bitcoin.