A private company would never allow the price of the product go below its manufacturing cost and, I think, the same can also be said to miners. Collectively, they will never allow the price go below the mining cost.
Explain how a miner can prevent the price from going below their cost. How does that miner stop everyone else from selling their bitcoins below his cost? How does that miner force buyers to pay more than his cost?
Also, the idea that nobody will ever sell an investment at a loss is ridiculous. People sell things at a loss all the time. I recently sold a bunch of stocks a loss, and I was happy to do it because not only will it reduce my taxes, but I will buy it all back later this year after the price has dropped even more.