I'm licking my lips at the prospect of putting together an ML rig from all the RTX 3090s hitting the market.
I get the point you're making, and would add that some ML applications bring tangible benefits to humanity which are proportionate to their energy consumption.
You'd need a "proof of ethics" and that seems technologically dubious.
Or, you'd be using blockchain to create an ethical application, and whatever benefits are realized by the application are probably not best provided by blockchain.
Using computing to compute meaningful data is directly justified or at least directly measurable.