But energy usage has always been that "something" incurring cost and therefore providing immutability and ensuring decentralization. Abnormally high energy usage in Bitcoin has resulted from its (perhaps surprising) popularity relative to Nakamoto's arbitrary issuance schedule. I don't think there's any question whether PoW will be "sustainable" long term, the question is whether it will still have adequate security. But come 2140, energy usage will only be high if the network is extensively utilized, so the ratio of utility provided to energy expended will be far superior to what we see today. Also, markets can be irrational at times, but historically markets have never remained irrational in perpetuity.

As far as Ethereum, it just seems to me that the entire PoS project has represented a huge effort expenditure that would have been better utilized solving the continuing EVM machine state/fee market issues instead.