double SHA-256 proof-of-work isn't computationally useful for anything
You don't find bitcoin useful? I disagree.
Well, computationally un-useful for things besides Bitcoin; that's what I meant.
Well this is hardly a criticism of blockchain tech, wouldn't you agree? Consider this criticism of headwear tech:
A helmet is only useful for protecting the head. It is un-useful for things besides head protection.
The criticism is a species of the "Bitcoin-wastes-electricity" criticism.