>Can we agree that people should not be able to unilaterally take existential risks with the future of humanity without the consent of humanity, based solely on their unilateral assessment of those risks?

No, we cannot, because that isn't practical. any of the nuclear armed countries can launch a nuclear strike tomorrow (hypothetically - but then again, isn't all "omg ai will kill us all" hypothetical, anyway?) - and they absolutely do not need consent of humanity, much less their own citizenry.

This is honestly, not a great argument.