One can make the same argument about anything. "Look at all the fabric wasted on beanie babies that could be used for clothes" one could have argued in 1998. if Bitcoin is just a fad ,energy use will plunge due to difficulty decreasing. The energy consumed on mining represents a an input cost based on an expected return on investment, like energy spent on car manufacturing, for example. if the return falls, so will input costs because no one operates at a loss for long.