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?
Electricity doesn't produce CO2, its production does and not even all of it because solar panels don't produce CO2.
Miners don't produce CO2 just like your home computer doesn't do it.
Would CO2 not be produced if we stopped mining? The answer is no.