I agree that it's short; now added this as a descriptor above. Technical report was the most appropriate category; they're usually longer.

... my immediate reaction from looking at the first two definitions given is that they don't rely on a specific moral framework or class of moral framework whereas the proposed definition seems to rely on some utilitarian or close to utilitarian notion.

We address this, saying:

A lot of the work of this definition is being done by the final couple of words. ‘Value’ refers simply to whatever it is we care about and want in the world, in the same way that ‘desirable future development’ worked in Bostrom’s definition.

What counts as an existential catastrophe does depend on the moral framework (which seems appropriate), but doesn't seem tied to any specific one. I agree that the simple definition (extinction) dodges anything like this, and that that is a point in its favour.