In some sense that's true, but missing the point. The amount of energy worth buying to mine crypto is exactly equal to the value of the crypto mined. What we should expect to see[1] is that the value of "Proof-of-Work ETH" (which is still a functioning blockchain[2], just like Ethereum Classic is) will drop as attention is focused on PoS ETH. And so energy devoted to it will drop in tandem.
It's also true that there are second order effects, like for example all the mining hardware dedicated to ETH needs to find a new home, which will depress prices for new mining hardware for "chains that are hardware-compatible with old ETH", and thus probably support their prices a bit.
[1] And do, I think. IIRC there was a stat rolling around a few months back showing electrical grid usage dropping due to the crypto crash, but can't remember where it was or how reliable the source seemed.
[2] Though AFAICT no one is tracking exchange rates for it yet, so your guess is as good as mine as to its value.