I'm not even sure that mining being expensive is a great defense against a 51% attack. If things do end up being an oligopoly, then any one of the members of t

I'm not even sure that mining being expensive is a great defense against a 51% attack. If things do end up being an oligopoly, then any one of the members of that oligopoly will only have a relatively small sprint between their current position and a 51% attack. And quite a few options for how to attempt such a sprint.