It's just an externality that can change at any given time. I guess it's only a problem if you for some reason believe sha256 will not be commoditized. The fact that energy costs keep becoming an ever higher percentage of the price per Bitcoin instead of hardware seems like it won't be a problem.
I see no evidence of that. In the GPU mining era, there was no extraordinarily efficient mining as there is now with the most efficient ASICs. All Bitcoins had approximately the same energy input. Now there a steep curve between the least efficient but still-viable ASICs and the most efficient. The area above that curve is missing energy. As far as I can tell the energy density of Bitcoin has
decreased.
I calculated the upfront hardware costs vs energy costs as a percentage of 1 Bitcoin between when that big Spoondoolies unit was originally released and also around a year or so before it.