One entity owning 70%+ of the hashrate is not centralization? What happens after the fork? One entity has 100% hashrate. This is like fiat. An entity has the power to print money.
Can any miner prevent others from mining also? They can't. That's why there is no centralisation. Central banks are because they are only entity that have ability to create money.
The person with the power can can orphan new miners blocks.
With central banks, they are only entity having this power, no one else has it. In Bitcoin this is not the case. That's why Bitcoin isn't centralised even if only 1 person is mining it
