I can only see the energy usage growing further. Because reducing the watt per GH/s will not be infinite, so far we've seen exceptional reduction <1w per gh, but we will reach a stage or phase where the reductions are so minuscule, that the overall mining power usage will grow as more miners are added.
And at that stage the economics will force miners to decentralize and create efficient heat as a product with PoW as a free byproduct to subsidize the energy bill.