There is a rational explanation. I think the number of bad actors cannot exceed majority as such a case would cause the network to break down hence eliminate all future cash flows to the miners.
So in evaluating the extra 15% return the decision tree involves weighing the future benefits from continued mining against the present 15% upside. Large miners wouldn’t do that unless they can switch to another network should the shit hit the fan… fringe mining pools could get away with this, which is what happened.
I am assuming ETH mining pools have their machines optimized for ETH mining and will be handicapped by screwing this network and switching to other networks.
The whole situation actually validates the security assumption of PoS, that players invested in the system have no monetary interest in sabotaging the network. If miners at ETH can’t/won’t do it even though they have choices then it is safe to assume that no sane Cardano stakeholder would either.
The things you learn from other people’s mistakes.