In my limited understanding, this is not an issue because it is in the miners' best interest to actually do their job correctly. They wouldn't have an incentive to muck around.
For the other cases like smart contracts and such maybe it's a little different. And perhaps that is why Ethereum is moving to Proof of Stake instead?
Well, they wouldn't have in-game incentive to muck around, but they could have meta-game incentive. Here's a good article explaining that:
https://news.ycombinator.com/item?id=21058809 (Rationality is Self-Defeating in Permissionless Systems).
What is your opinion on this topic?