Electricity produces CO2,but so what?Trees and plants consume CO2,so the problem with CO2 production can be solved,if we plant more trees on the planet Earth.
Can you measure "environmental sustainability"?I don't think so.
Coal mining and coal consumption in the third world countries is a way bigger problem than crypto mining,so let's not become like all those "progressive" ecological fanatics,like Greta Tunberg.
Many industries are polluting the Earth way more than the crypto industry.
And yes,crypto mining will become more and more energy efficient with time.