At the present bitcoin doesn't have any defense against a single new miner with 51% overtaking the network.
That isn't necessary.
How could bitcoin defend against itself?
This had been asked to Satoshi at the design of this project. It is more profitable for the miners to sell than to invest all only to bring down the network. If the mining pool gets too large miners could switch to another pool. This behaviour has been seen in the past with large mining pools.