PoW should actually stand for Proof of Waste. It's not even unprecedented in nature: many species will intentionally waste energy or resources to demonstrate mating fitness, as with a peacock's tail.

https://en.wikipedia.org/wiki/Signalling_theory

... or people going to the gym.

The problems start, of course, when you take a concept to its logical extreme.

I'm less concerned with wasting resources (which is highly subjective), than with ecological and systemic harms. I don't care whether a terawatt is "wasted" on pointless SHA256 hashes, or calculating triangles for Yet Another Marvel Movie, so long as the externality is being paid for [0].

[0] https://en.wikipedia.org/wiki/Pigovian_tax

Exactly. And by forcing the internalization of that externality, that forces the market to consider things which lack that externality.

The trick is to enforce it in such a way that it can't be easily dodged via e.g. offshoring.