The problem isn't viability, it's the fact that PoS re-centralises wealth for almost no cost to the minter which makes it inferior to PoW. The problem with PoW is the green aspect, of course
Oh wait, most mining operations are in the millions of dollars range, nevermind the fact that 84% of all BTC is held by .3% of addresses with > 100 BTC...
Both PoW and PoS have centralisation, but PoS vastly increases this centralisation, does it at a far faster rate, and "locks it in" (there is no feasible way to overcome the most powerful node in PoS, this is not the case thankfully in PoW)