What are the checking mechanisms that 2/3 of the nodes are not colluding together? In other words how do you prove someone owning 51% or 2/3 of the network?
Exchanges hold the most ETH. They are a central collusion point which could cause undue influence on the network.
At least it seems they do not own a lot of the existing Eth.
The main goal of crypto - the base token and protocol rewards from PoW or PoS - is to secure this network and keep it permissionless and decentralized.