There may be other solutions or defence mechanisms, but one solution would be to fork again and remove the Ether those validators have staked from your chain as part of your fork.
A chain is defined by a set of rules that everyone on the chain agrees to abide by. Braking the rules damages the economic utility of the chain, so the Ether on the chain where the validators colluded to break the rules would be worth less, as a result of their actions. The validators are locked on that chain (as they can't prevent a fork from excluding them), so they have a strong incentive against harming the chain. Ethereum is not majoritarian. I don't have to abide by the will of the majority. I only need to come to a consensus about the rules and state of the chain with the people with whom I want to share the chain.
PoS makes it much easier to escape a malicious majority compared to PoW where the hashing power majority can follow the minority anywhere unless the minority is willing to switch to a different hashing algorithm. Even then, the majority could sell their mining equipment and buy new equipment that will work on the new chain on the same terms as that chain's honest minority can. With PoS, the malicious majority would need to buy new Ether from the honest minority who can then just fork again after having made a profit on the attack, and can continue to do that until the malicious majority runs out of resources or realizes that they cannot censor a fork in Ethereum's PoS system.
> Braking the rules damages the economic utility of the chain, so the Ether on the chain where the validators colluded to break the rules would be worth less, as a result of their actions.
So you’re going to create a new fork, remove the money from the majority of the wealthy stakeholders who are governing the system, and expect this one to be be seen as legitimate? The one that is explicitly giving the finger to the wealthy?
I don't think legitimacy is a meaningful concept in this context. I would use the chain with the greatest utility, which is determined by whether I like the rules, whether the rules are consistently and predictably enforced, and whether other people I want to interact with are using that chain. Whether the people who use the chain have more Ether or more computer power compared to the people who use a competing chain doesn't affect utility.
While I don't think it's meaningful to say that one chain is more legitimate than another, if I was forced to make such a deamination in this case, I would consider the chain where the rules are consistently and predictably enforced in a way that leads to predictable outcomes more legitimate than a chain where validators have colluded to arbitrarily subvert the enforcement of the rules.
A chain is defined by a set of rules that everyone on the chain agrees to abide by. Braking the rules damages the economic utility of the chain, so the Ether on the chain where the validators colluded to break the rules would be worth less, as a result of their actions. The validators are locked on that chain (as they can't prevent a fork from excluding them), so they have a strong incentive against harming the chain. Ethereum is not majoritarian. I don't have to abide by the will of the majority. I only need to come to a consensus about the rules and state of the chain with the people with whom I want to share the chain.
PoS makes it much easier to escape a malicious majority compared to PoW where the hashing power majority can follow the minority anywhere unless the minority is willing to switch to a different hashing algorithm. Even then, the majority could sell their mining equipment and buy new equipment that will work on the new chain on the same terms as that chain's honest minority can. With PoS, the malicious majority would need to buy new Ether from the honest minority who can then just fork again after having made a profit on the attack, and can continue to do that until the malicious majority runs out of resources or realizes that they cannot censor a fork in Ethereum's PoS system.