So, I guess the question is, what is the feasible price of bitcoin when considering other objective functions such as environmental sustainability?
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,year after year.The miners want lower electricity costs,don't they?
And the biggest problem is people's fallacious and even destructive thinking that since there are much bigger problems out there the smaller ones could just be set aside and not attended to. In the first place, those bigger problems which are now seemingly insurmountable was but a small problem back then.
In a way, environmental sustainability could indeed be measured. That is why a set of criteria is integrated in business license applications or even project proposals that the planned business or project does not cause terrible harm to the environment.