What happens if the price drops below what is profitable to mine? Do they scale back or stay the course because if other miners back out they’re more likely to be rewarded a block? I guess there’s a 51% attack vector there too if the price drops too much and enough miners stop mining?
The difficulty will adapt to the slower block minting times, and those who stick around, will become profitable again. This is all written in the protocol.
In theory, the mining rate scales back accordingly. In practice, the marginal cost of continuing to mine is probably very low: miners have made the upfront investment in the hardware, and many of them are located in areas where they're receiving artificially cheap electricity (or have reopened coal plants solely to mine[1]). In other words: large mining groups probably aren't feeling anywhere near enough pain yet.
Economically, a 51% attack is unlikely: it would fundamentally compromise trust in the network, which would probably lead to selloffs and a crash.
