Is it fanciful to hope this could - in and of itself - allow several polluting power stations to be decommissioned, and/or appreciably reduce energy prices for the rest of us?
That math is going to change as the Ethereum miners reconfigure for altcoins and the difficulties change, but the bottom line is that mining will continue as long as some coin is still profitable to mine.
We'll see if it plays out, but cryptocurrency prices falling have certainly reduced the energy consumption as well (it becomes less profitable to mine at a given difficulty when your mining reward is worth less).
If Ethereum does eventually manage to overtake Bitcoin, that'll bode well for crypto's energy footprint in the long term (fingers crossed for this happening, or at least for there to not be another Bitcoin/PoW bubble)
Given this, a LOT of mining capacity is going to go offline when this occurs.