If bitcoin stopped existing today, the environment would be better off because right now the impact is negative. It could only become net-positive, if there are changes in the future. Even if you believe such changes will happen, I think you'll have to agree that this is speculation. It's not net-positive for the environment, it might be net-positive for the environment.

That's unrelated to it being negative-sum though. Running the numbers on this is trivial. You can only profit from bitcoin by selling it to someone else for more than you paid for it. Inevitably for some people to profit, some people have to lose a corresponding amount. This alone would make bitcoin zero-sum. Some people, miners, are special though in that they are by design always making a profit. That they extract from the network making bitcoin negative-sum.

As a consequence it's positive only for those people who find a loser to sell to, are miners or "shovel" makers (electricity, hardware, ...)