So they let the miners sit unused while mining is unprofitable? If so, that's annoyingly inefficient... Surely there's something a miner can do other than mine, during these periods..

Those miners represent a capital investment. They could recoup some of that investment by selling the rigs. I'd argue that ai compute & bitcoin mining incentives will constantly compete for energy usage & capital allocation. What might not be sustainable today may be highly profitable in 3 months.