I always see people point to 'just round the corner' future Proof of Stake systems when the energy waste topic comes up, but it seems like there are still a lot of big unsolved challenges with that, as well as new problems introduced by it.
As long as there's no actual widespread fully working currency using only PoS, it's a bit like justifying the emissions from flying by pointing to experimental solar powered aircraft, and saying all flights will be solar powered any day now.
I keep seeing discussions of this include something like 'but it will all be moving to proof of stake soon', which seems to me a bit like arguing for how green air travel is by saying we'll have solar powered airplanes soon.
A lot of faith seems to be being placed in an unproven advance, which if it is really possible at all (and that's a much bigger if than many acknowledge), would introduce all sorts of new problems.
(the link was there but buried at the bottom of the page)