Maybe I'm missing a key detail here. Let's say Alice, Bob and Carol (and no one else) are all mining bitcoin. For simplicity, one block is mined per day, the

Maybe I'm missing a key detail here. Let's say Alice, Bob and Carol (and no one else) are all mining bitcoin. For simplicity, one block is mined per day, the block reward is worth $300, and so the system is in equilibrium when they all spend $100 a day on electricity. Difficulty adjusts every 10 blocks. Now the value of bitcoin drops precipitously - let's say they wake up one day and the block reward is worth $3. Let's say they all shut off their machines. Dave wakes up and sees no one is mining bitcoin, and thinks "I can be the last one standing". He now has to spend $300 * 10 = $3000 (in the worst case where the drop is right after the last adjustment) to mine ten blocks, for which he receives $30 worth of rewards. But how does this help him? If the difficulty adjusts downward, Alice, Bob and Carol can all just start mining again. It's not like they're permanently locked out of the system because they turned off their mining rigs. So Dave has spent net $2970 for...what exactly? The ability to run a 51% attack and double spend to recoup his cost? To run the double-spend, he has to re-mine a second side chain at the way-too-high difficulty! I'm skeptical that this is an attractive proposition for Dave.