That won't be Bitcoin anymore. It would be just another centralized fiat with the same fundamental problems that led to creation of decentralized bitcoin with a capped supply.
It makes no sense to destroy Bitcoin like that!

Once you make the statement that bitcoin can be a centralised currency, it only means that you've started to miss it at that Very point and you're going away from what bitcoin actually is and what has kept it standing even till the present. Regardless of the mass adoption of bitcoin, there won't be a bitcoin bank or a central point where bitcoin transactions are being monitored and in a way regulated. if that happens, then it practically means that bitcoin has lost it all and isn't different from the fiat.

Bitcoin can become a world currency without being centralised and the concept should be that people across the world makes use of bitcoin globally for all her needs and even without a central body literally being at the centre of it. We are all positive about the mass adoption of bitcoin but at the institutional stage and it use as national reserve but we are not hoping that when that eventually happens, it should cause bitcoin to lose what it actually stands for.

The world is even getting more digital and a lot of the things that are not yet in the digital form and going towards that direction. We are approaching a cashless economy and with time, fiat won't be too much in use because a lot of businesses would prefer to carry out her transactions on a digital bases. If the fiat can be going in that direction, how much more bitcoin that is built to function that way?

Yep, it would become Centrecoin, not the one coin we love and cherish

if bitcoin turn to world central currency and become centralize as you suggested it will no longer be a store of value with potential returns anymore, because everything about bitcoin will be manipulated since there will be a central authority that will be in control of bitcoin, so in my own opinion in other for bitcoin to maintain it current and normal state, bitcoin should remain decentralize as it has always been for years now.

You have tried to find a way to increase the use of Bitcoin. You have tried to find an alternative method so that people from all walks of life can easily use Bitcoin for all purposes. I respect your thinking.

But we should not forget that the reason why Bitcoin is so popular is its decentralized system. Everyone in the world wants to own Bitcoin. But it is not so easy to hold it or keep it for yourself. Because people get scared at the slightest moment or sell their holdings in the hope of making more profit in a short period of time. So if Bitcoin wants to have its own banking system, then its volatility and decentralized behavior will be destroyed.

If you look at the characteristics of Bitcoin, you will see that these two natures are the most powerful and reliable. So Bitcoin does not need any bank. If anything is needed, it may be to reach the public with true, accurate and reliable information about Bitcoin.

We should all empower ourselves and others to know the right information about Bitcoin.

We would lost it all. Freedom, decentralization, anonymity, cross border payment. It means going back to square one. Bitcoin was never created to be a centralized currency for government to control, but to be the opposite of fiat and that's why you see bitcoin where it's today. Anything that the government put their hands into becomes a tool to manipulate the country.

It's not compulsory for everyone to adopt bitcoin, but it's already a world currency which you can use as a payment method provided that it is not illegal over there.

Satoshi, created Bitcoin and disappeared to proof that Bitcoin doesn't need an owner or a centralized authority to own it. The essence of Bitcoin will be lost if it where ever to be printed like a physical cash, then it'll have a central bank to monitor and regulate it. What makes Bitcoin to be different from fiat is that it is decentralized and have a limited supply, it goes against everything that fiat represents. Bitcoin don't have to be printed as physical cash to become a global currency, it's adoption is increasing very fast and in years to come I believe that p2p transactions will become very common like fiat.

Bitcoin will not become a centralized currency for global use.

Read the bitcoin whitepaper again by Satoshi and you will understand that it was never meant to serve that purpose. Just like we cannot have one unifying language we cannot have a central currency.

If assuming for example that it happens, it will change everything written in the whitepaper.

This is the abstract below to give you context

There is no need for that. We're already the bank for Bitcoin and if there will be a bank that's going to adopt Bitcoin then they're free to adopt that and make services out of it. I think the common service that they're going to do and adopt is about loaning people who owns Bitcoin and that will be the collateral and their credit limit/score will be dependent on how much they hold. So, the more BTC the holder has, then they're sure that this person is going to pay them the debt that's taken and applied on them. Because before, we holders are having hard time getting services in the banks because they're doubting Bitcoin. But who knows soon, if we'll be the most sought customer for these credit lines.

SO you are advocating Bitcoin becoming centralised - which does not make sense at all.

The very core of Bitcoin is that it IS a decentralised "world currency" and with that is the fact
that no one central agency or country controls it and in turn is able to manipulate it.

Every centralised FIAT currency has at some point in its existence been manipulated by its
controlling bank and or country.

As others have pointed out Bitcoin is already a world currency, it can be used* anywhere in
the world. *bought, sold and transferred

Anyway, being a world currency in order to buy a coffee is not necessarily what everyone wants.