I have used both and i come to know that native currencies are also very important and we can not say that native currencies are not important we have seen that native currencies are now using for transactions through banks and in future blockchain will take the place of fiat currencies.

Native currencies has many more users than the cryptocurrencies and it will a lot of years before bitcoin can match the total number of native currency users. People spend more on their fiat than in bitcoin or any cryptocurrency so it is not possible for the virtual coins to reduce the value of native currency. There are alsoseveral factors that really affect the value of the native currency like the surge in demands of foreign goods and trade flows, speculations, among them. Bitcoin must be use widely first before it can affect any national currency. 

Technically yes as the digital currency gets popular people wont bother to use the old banking system like fiat because it's less secure and slow where you can just send lighting speed transactions around the world with no restrictions like regulations. But this will take time because we've not yet overcome the disadvantage of digital currency.

if in my opinion. that's for sure bro. especially in countries that have accepted bitcoin as a legitimate currency such as paper currency. but if in Indonesia. the government still prohibits bitcoin from being used as a tool to buy in the real world.

I don't think it will reduce the value of real currencies, because the use of bitcoin as a digital currency remains not as much as the use of real currency. In addition, bitcoin is used by the upper middle class who master technology where not all countries legalize it.

it seems like it doesn't, if more and more are benefiting bitcoin like it doesn't really have a big influence on the price of the original currency, and like that we still need fiat to transact in crypto and in the real world. and what I'm still confused about is why the BTC price still goes down even though it looks green even though it's a little.

Of course, bitcoin can reduce the use of Fiat money. But it won't happen because governments won't let it happen. Bitcoin has many advantages over Fiat money and if it is well embedded in our lives, the use of Fiat money will be useless. Therefore, I believe that bitcoin will exist as a second currency.

How can it possible because bitcoin and fiat currencies are two different... Bitcoin which is controlled by blockchain and fiat money which is controlled by bank and government.

I dont see this happening soon . Cryptocurrency and Fiat are somewhat dependent on each other. youll need to convert your Cash to Crypto to use cryptocurrency and Crypto back to Fiat to use fiat in your country.  
Stabecoins are pegged to fiats, by the way. This i believe will help fiats greatly
 Maybe things will change in the future when p2p transaction with Bitcoin/Crypto becomes common in most countries