No, its not centralisation. One can create other shitcoins or split the chain but needs to fight it out with SV miners, by putting in their own money where their mouth is & doing PoW
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.
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