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?
It is better if we care for ocean algae because they are the biggest CO2 absorber, planting a lot of trees will be inefficient because maturity takes a lot of time. If we do not solve the electricity consumption problem within crypto mining then what is it good for rather than contributing to the carbon footprint. Can you explain how the mines become more energy efficient?
There is a solution for this problem that we are facing, as we all know, more countries are leaning towards a more efficient and clean energy like wind, solar, hydro and the worst but efficient of the bunch which is nuclear. The thing with power source is the farther you are to the source, the more expensive the cost of electricity is. If we can put those mines as close as possible to the source, I think that we can alleviate the environmental impacts that mining contributes.