At what cost? Forking reduces the security since its subsidised by the tokenomics that is the incentive to hold the data for others. Also if you fork why waste resources holding data you don't care about and thus prune the blockchain you fork
As the chain gets bigger it reduces the amount of people with the resources to do manage and you always have to trust a set of witnesses, moving the trust assumptions to a different party at great cost doesn't remove trust
In a situation where a chain has been corrupted by nefarious witnesses, the choice to not fork is the one that reduces security. I'm not sure why security would be reduced due to financial incentive in the way you are stating.
Whoever runs the fork can do so in whatever way they prefer. If the users don't like the way the fork is setup they can fork their own version.
If the processing requirements exceed the level of hardware that can be procured by the amount of rewards paid to the witnesses, then mitigation can be put in place that involves sharding, pooling or other strategies. However, we have never reached a point where this has happened.
I have already addressed this in previous comments.