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.

Cardano doesnt even have smart contracts yet, how have they implemented nfts?

otherwise yes, energy usage will be dropping massively on most networks with pos upgrades.