It's good to be looking for holes in the #Bitcoin and PoW system. But I don't see it. We are stretching. EVEN if the pools of miners become more concentrated, LONG before their is a problem there will be a LOT of discussion.
The miners have no incentive to corrupt the network. EVEN if miners get to a 51% attack (which we will know long before it happens) there will be 1000 working on fixing the problem due to their OWN interests. Let's not forget the Nodes - which I run, along with at least 20,000 others.
The system will only grow stronger against attacks, as it has done sine the block size wars.