When mining is not profitable (such as just after a halving for a miner who was already on the edge of profitability), miners leave the market and the difficult

When mining is not profitable (such as just after a halving for a miner who was already on the edge of profitability), miners leave the market and the difficulty rate adjusts until the market is once again in equilibrium. Transactions do take energy, but currently only a fraction (about 1/7) of the mining reward is paid for by transaction fees alone. An interesting question is whether the network can stay secure if the difficulty drops to ~1/7th of what it is today, or whether something else will have to give. (For what it's worth, even at 1/7th of the energy use the network is still incredibly wasteful by design.)