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.