I don’t understand why “death is negative” needs to be an axiom. For me, the reason death is (usually) negative is a consequence of “suffering is bad.”
If the entire solar system would instantaneously blip out of existence (without anyone knowing about it beforehand), I don’t see why that would be bad. I agree that thinking about this scenario causes mild suffering, but IMO that doesn’t imply that the event itself is bad. Just that we sometimes have flawed intuitions.
Opportunity cost and value require a subject for whom things can be better or worse. In a lifeless universe, there are no such subjects, so nothing is lost.