Reading some information on POS from
https://ethereum.org/en/developers/docs/consensus-mechanisms/pos/. Here is their explanation about the security of PoS against 51% attack.
The threat of a 51% attack still exists on proof-of-stake as it does on proof-of-work, but it's even riskier for the attackers. A attacker would need 51% of the staked ETH (about $15,000,000,000 USD). They could then use their own attestations to ensure their preferred fork was the one with the most accumulated attestations. The 'weight' of accumulated attestations is what consensus clients use to determine the correct chain, so this attacker would be able to make their fork the canonical one. However, a strength of proof-of-stake over proof-of-work is that the community has flexibility in mounting a counter-attack. For example, the honest validators could decide to keep building on the minority chain and ignore the attacker's fork while encouraging apps, exchanges, and pools to do the same. They could also decide to forcibly remove the attacker from the network and destroy their staked ether. These are strong economic defenses against a 51% attack.
51% attacks are just one flavor of malicious activity. Bad actors could attempt long-range attacks (although the finality gadget neutralizes this attack vector), short range 'reorgs' (although proposer boosting and attestation deadlines mitigate this), bouncing and balancing attacks (also mitigated by proposer boosting, and these attacks have anyway only been demonstrated under idealized network conditions) or avalanche attacks (neutralized by the fork choice algorithms rule of only considering the latest message).
Overall, proof-of-stake, as it is implemented on Ethereum, has been demonstrated to be more economically secure than proof-of-work.
Take note of the bolded part. That would be a strong warning to any exchanges wanting to take control of the ETH network. So I guess exchanges will not try to control ETH when it becomes POS.
Damn, now I realized how centralized PoS ETH will be.