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
So there is no wealth centralization in BTC or ETH?
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...
You'll always have centralisation in any system, you're just trying to minimise it as much as possible. Mining operations require you to continuously supply to the system at cost, which is completely different to staking that requires extremely minimal to no cost for operation.
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)
I'll believe it when I see it, but it looks like it's been in the pipeline for almost 6 years[1], so I'd be very surprised if we see it before the crypto bubble bursts.
How? mining new coins doesn't cost energy, preventing double-spends does. Premining wouldn't make any difference
Proof of stake or some other non proof of work consensus mechanism could reduce the energy usage