Comments
This is the wrong thing to try to figure out; most of the probability of existential risk is likely not to make a clear or intelligible story. Quoting Nick Bostrom:
Suppose our intuitions about which future scenarios are “plausible and realistic” are shaped by what we see on TV and in movies and what we read in novels. (After all, a large part of the discourse about the future that people encounter is in the form of fiction and other recreational contexts.) We should then, when thinking critically, suspect our intuitions of being biased in the direction of overestimating the probability of those scenarios that make for a good story, since such scenarios will seem much more familiar and more “real”. This Good-story bias could be quite powerful. When was the last time you saw a movie about humankind suddenly going extinct (without warning and without being replaced by some other civilization)? While this scenario may be much more probable than a scenario in which human heroes successfully repel an invasion of monsters or robot warriors, it wouldn’t be much fun to watch. So we don’t see many stories of that kind. If we are not careful, we can be mislead into believing that the boring scenario is too farfetched to be worth taking seriously. In general, if we think there is a Good-story bias, we may upon reflection want to increase our credence in boring hypotheses and decrease our credence in interesting, dramatic hypotheses. The net effect would be to redistribute probability among existential risks in favor of those that seem to harder to fit into a selling narrative, and possibly to increase the probability of the existential risks as a group.
I'm writing a comment and not an answer because I think this also doesn't meet your criteria (too long, I'd guess), but I thought I'd mention It Looks Like You’re Trying To Take Over The World, a short story written by Gwern that is on this theme.