Miners invested in equipment. That's more than electricity. Some get electricity for free or flat. So they'll keep mining.

But you are right - should the price drop extremely low, we may never make it to the next adjustment of difficulty. But frankly I can't see why the price would drop to sub $1 levels. As plausible as gold dropping to sub $1 levels / ounce.

For reasons of stability and low latency in transactions, the network tries to produce one block every 10 minutes. Every 2016 blocks (which should take two weeks if this goal is kept perfectly), every Bitcoin client compares the actual time it took to generate these blocks with the two week goal and modifies the target by the percentage difference. This makes the proof-of-work problem more or less difficult. A single retarget never changes the target by more than a factor of 4 either way to prevent large changes in difficulty.