Efficiency in energy may be one thing but the eventual factor will be cost.
In the end, miners will find a new equilibrium between hardware and electricity costs on the one hand, and block rewards on the other hand. If hardware becomes more efficient, they'll buy more hardware and consume more power again. And if they don't do it, someone else will.The only way to really use less energy, is if the mining hardware is so much more expensive that it takes a larger share of the total costs, which leaves less money to be spend on electricity. But I wouldn't be surprised if most of the energy savings are consumed again by the hardware manufacturer. It's not as if CPU production doesn't consume a lot of energy!
And we're also talking about renewable resources here. I daresay that these new computing hardware that's always aiming to be more efficient (energy and space wise, usually) are requiring more and more material that's scarce - rare metals, etc. These are getting more and more expensive, so CPU production not only consumes a lot of energy but a finite source of rare material that will get even more scarce as production levels multiply - and along with that, demand.
Like you said, though, equilibrium is always eventually reached and there's still so many variables that could change during the timespans. Not least Bitcoin price.
