Earlier quoted context omitted.
The environmental impact. And yes, I know, 0.5%, but my issue was always that if PoW currencies went from being a niche subculture to a point where it was used for everyday exchange (many people were arguing that this would and should happen) that 0.5% would surely go up by a great deal. To a point where crypto had to clear a super high bar of usefulness to counterbalance the harm it would do. To be fair, AI training…
There is no "environmental impact". Environmental impact comes from energy production, not energy usage. It's incoherent to argue others should tamper down their energy usage because most folks producing energy aren't doing it in an ethical way.
There's a general consensus that paying someone else to do your dirty work doesn't free you of the moral (or, usually, legal) culpability for the damage done. If you knowingly direct your money towards unethical providers, you are directly increasing the demand for unethical behavior.
(That's assuming that the producers themselves are responsible for the ethics. If a producer is doing its best to convert to clean energy as fast as possible, they may be entirely in the clear but POW would still be unethical. In that scenario POW is placing strain on the limited clean energy supplies, forcing the producer to use more fossil fuels than they'd otherwise need to.)