Sunk cost fallacy. Once you have mining hardware all you care about is variable costs. Mining hardware costs only affect whether you buy more hardware.
But is it profitable to buy hardware, knowing that it will only run some of the time, when the mempool gets extra full? Maybe. Depends on both the hardware cost and the variable costs.In any case, I agree it will be more profitable to mine when the mempool is bigger, but the *degree* to which miners will smooth over block-time variability depends on their costs.
It does seem inefficient for miners to turn on and off so frequently
