First, when the network space is growing exponentially, the value of preexisting work is decaying exponentially. I do grant you that completed work is nonzero in value, but during a growth phase its value might as well be zero. Whenever speculators believe the currency will actually have any long term value above its current exchange rate minus the resource costs, then it will be in a growth phase. The total resource consumption during this growth phase, including the supply chains to produce space, need to be studied. Measured in transactions per unit of CO2 emissions, I expect its efficiency to be better only during a phase of stable network space, i.e. when its value is not speculated to be higher than its current value.
Second, not all PoW is discarded. I want to be clear at the outset that this isn't a defense of PoW, but there's a system called AuxPoW which allows work to be applied to multiple chains, modestly improving its efficiency. This mining protocol is being used actively by scrypt hashed coins, e.g. dogecoin and litecoin. I'm certainly not saying PoW is superior to the proof system in Chia, but just that the distinction between them is not as strong as claimed. Nevertheless, both are incredibly destructive and yet interesting.