The proof of work needs to be “useless” in order for it to protect the blockchain (there are proposals that dispute this, and I won’t claim to be an expert, but that’s the fundamental idea behind Bitcoin).

When you say that the net value is negative even if the network itself has some positive benefit, what is the negative component there? The only negative component of all that electricity usage is externalities, which apply equally to each unit of energy regardless of what it is used for. Pricing externalities into the cost of electricity is obviously something I approve of, but that’s a separate issue, and I doubt the externalities of electricity usage are anywhere near the magnitude of the positive impact of bitcoin.