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.
The algorithm for Bitcoin Difficulty change is not one way. If miners stop mining and the number of blocks solved per hours drops below parameters, the difficulty will adjust downward to keep the number of blocks solved per hour within parameters.
Thats not the point - the thing is that difficulty does not get adjusted after 14 days. It gets adjusted after 2016 blocks. Should the hashrate drop significantly, the time to the next downward adjustment will also be very long. During that time, TX processing can be so slow that it affects Bitcoin price. That may make the remaining miners give up mining. TX processing would come to a halt and Bitcoin would die.
This scenario isn't realistic, though - at least those miners who don't pay for power will keep mining. As will those miners who simply want Bitcoin to survive.