so milk price how many btc? fuel price? rent ? and how the prices will calculated?
Milk, fuel, rent, etc. will be priced in BTC the same way it is currently priced in your currency. How would you price milk in euros or yen or Canadian dollars? There is no difference except the currency.
and if there is only limited supply of bitcoins.....then those who who have bitcoins merely will rule over others?
so your skills or value in society dont count only matters who go the most of the bitcoins?
If Bitcoin is the only currency then everyone will have it. Nobody can use it to rule over others because it is censorship-resistant and it is not controlled by anyone.
bitcoin halving by the time so less and less btc will be in circleation....so how the world will remain sustaineble then ?
The halving does not reduce the number of bitcoins. Bitcoins are not consumed. The number of bitcoins increases until it reaches 21 million. It doesn't go down.
okay if bitcoin is not good to be currency then ethereum ?? well ethereum is centralised as i know ?
Ethereum is not centralized.
stable currenies?? well i dont see point of them they are just the 1:1 of the same fiat currrencies.
If there are no fiat currencies, then there will be no coins pegged to fiat currencies.
so where does the theory come from then we go all in cryptocurrency and fiat currency will disapere?
well this is not possible bitcoin without fiat can not serve all the peoples currecny needs !!
What needs cannot be served by Bitcoin that can be served by fiat currencies?
How we are going to control the money supply how big quantity of currency is in circelation?
The money supply is controlled by the protocol --> The money supply increases at a predetermined rate until it reaches 21 million BTC.
if bitcoin is the future of money without fiat then everybody will have the guns and shoot each other:D just to get other bitcoins becouse btc has limited supply lol

If people want money, they earn it (or shoot people). Bitcoin doesn't change that.
so i want to know if fiat currency will disapere then how the new currency will be issued?
Bitcoins are issued in every block until there are 21 million.
and how it will be backed?
Fiat currencies are also backed by nothing. Bitcoin has the advantage of being more dependable and is not subjected to the whims of people and governments.