In practice, there are behaviours which provide no net social benefit, exact tremendous costs, and yet are inherently self-perpetuating. Legal prohibitions can help in this case. It's a form of demand-dampening.
It's only your opinion that there's no social benefit. Which is a problem, because your opinion might be wrong. If you charge people appropriately for their carbon use such that it's no skin off your back if they continue consumption, then you don't risk destroying value in places where your opinion turns out wrong.
I think you're on the wrong side of caution here, if we stop activities and so help to solve climate change, then we can look later at whether those activities people thought were just empty carbon producing were actually supremely beneficial. You can't make an omelette without breaking eggs, as the saying goes. Bitcoin/altcoins are certainly not vital to survival.
Cut carbon, then make a case for bringing back heavy carbon producing activities that seem to be of little use.
In practice, there are behaviours which provide no net social benefit, exact tremendous costs, and yet are inherently self-perpetuating. Legal prohibitions can help in this case. It's a form of demand-dampening.