"Unless > 50% of the miners, by hash capacity, make a change that affects the blockchain, it doesn't happen." This is not true at all. If the top 4 pools start

"Unless > 50% of the miners, by hash capacity, make a change that affects the blockchain, it doesn't happen." This is not true at all. If the top 4 pools start accepting and mining 20MB blocks, no one else in the network will accept the blocks, even if that chain is longer, because it would violate the fundamental rule, hard coded in their Bitcoin implementation, that blocks must be < 1MB. It would effectively fork the block chain.