> If the POW is maximally hard (energy intensive) then transaction fees will need to be high for miners to have ROI. If the transaction fees aren't high enough

> If the POW is maximally hard (energy intensive) then transaction fees will need to be high for miners to have ROI. If the transaction fees aren't high enough to support the current level of mining them some (but not all) miners will drop out (and the "difficulty" will adjust to compensate). The result is transactions will continue to be processed, but the overall security of the network will be lower.