What counts as an existential catastrophe does depend on the moral framework (which seems appropriate), but doesn't seem tied to any specific one.

Different frameworks can definitely disagree on whether some events are catastrophes. E.g., a new World War erupting might seem a good thing to some who believe in the Rapture.

If you're saying that some nontrivial subset of potential catastrophes are universally regarded as such, then I think that should be substantiated. If OTOH you saying this is true as long as you ignore some parts of humanity, then you should specify which parts.