The system is fault tolerant. You typically get 99.X% participating. Any validator that doesn't perform their duty in a timely fashion is penalized and eventual

The system is fault tolerant. You typically get 99.X% participating. Any validator that doesn't perform their duty in a timely fashion is penalized and eventually ejected if they are disruptive. Those private keys are useless unless you had something like 50% of all the active validators' keys. So, hundreds of thousands of private keys hacked. You're not going to be able to damage consensus using a few old leaked private keys. The best you could do would be to slash some active validators and get them ejected, but the chain would carry on finalizing without them.