The bitcoin blockchain relies on the assumption that the mining power is honest and no single entity will hold the majority of the mining power. This has proven

The bitcoin blockchain relies on the assumption that the mining power is honest and no single entity will hold the majority of the mining power. This has proven to be an issue with the selfish miner case and when a single mining pool reached 51% of the total mining power. In the world of security you simple cannot assume honesty and build security on top of this assumption.