Miners that mine at a loss should stop doing that, unless they want to speculate on a future price rise.
But what if that's the majority of miners and just breaking even miners? (You're probably right, but just want to be sure).
Also, big mining operations are not built on short-term, daily, price fluctuations. They have contracts on infrastructure rent and energy for several months, and they will...
...mine even with a short-term loss, speculating on a future price rise.
Difficulty, and with it profitability, of mining changes on every block. If some miners stop mining, difficulty and profitability will readjust for remaining miners. It self-adjusts.
Therefore keeping the network secure.
