Disclaimer: I'm not a bitcoin expert, so please correct me if I'm wrong.

With that said, here's what I think would happen:

1. Bitcoin price goes down enough that a lot of miners leave

2. The difficulty decreases since blocks are taking more than 10 minutes to produce

3. It gets cheaper to mine bitcoins, since less hash calculations are necessary per block (on average)

4. Miners want to join the system again, since it's cheap, thus taking away the risk of a 51% miner

So, bitcoin should self-correct