> which again brings in the subjective judging of whether the thing is important or not
Completely agree.
> far less than clothes dryers, and far less than gold mining (and gold mining is pretty horrible for human rights and land destruction as well).
Agree too.
> I would rather just tax the externalities and let people decide how to spend inside of that.
Mhm, I don't believe in the effectiveness of this and I think we need something stronger than this, but that's a belief. One problem with taxing externalities is that richer people get to waste more with fewer consequences than poorer people a.k.a they have a right to pollute more / more power to pollute and I don't think we should be able to buy "pollution power". Currently, the amount of money one have is actually already roughly equivalent to one's power to pollute. Limiting the amount of CO2 each people can spend (without the ability to transfer, because that's equivalent to being able to buy pollution power", equally, might be fairer, and that would probably eliminate PoW mining because people will probably want to "spend" their limited amount of pollution in other ways.
Of course, I'd even prefer that people reach the conclusion that PoW mining is heresy and decide by themselves not to do that. That's an opinion, of course.