If bitcoin's benefits do not offset the "massive costs" for you (because you're likely coming from a first-world country), it doesn't mean that these benefits do not exist for people who were not so lucky as you are.
Carbon pollution is not just a one-time cost, it is a debt that carries a massive interest rate with it that will be paid for generations. So whatever benefits someone else might reap from Bitcoin, it certainly isn't worth it to me.
People use way more energy for dumb heating. In my region humans can't survive in winter without it. There is no reason why mining rigs can't be used for 'smart' heating, heat being a byproduct of mining.
It is not done currently only because the energy use of bitcoin is so miniscule compared to other uses, that it's not even worthy to bother yet. Down the road, it'll likely change, but currently complaints about 'massive costs to the environment' are mostly virtue signalling.
If the energy use of bitcoin was minuscule while heating a building, and it could generate mining rewards while doing so, then people would be using it for that purpose. They aren't.
That's my point. It's not popular because it's not economically viable in many cases. So heating buildings is another thing that Bitcoin is supposed to improve, except it really doesn't.
Why do you think it would be less economically viable to use mining for rewards + heating rather than just rewards? It's both economically advantageous and arguably slightly less pollutive to offset energy consumption used for heating.