A motivating scenario could be: imagine you are trying to provide examples to help convince a skeptical friend that it is in fact possible to positively change the long-run future by actively seeking and pursuing opportunities to reduce existential risk.
Examples of things that are kind of close but miss the mark
- There are probably decent historical examples where people reduced existential risk but where thoes people didn't really have longtermist-EA-type motivations (maybe more "generally wanting to do good" plus "in the right place at the right time")
- There are probably meta-level things that longtermist EA community members can take credit for (e.g. "get lots of people to think seriously about reducing x risk"), but these aren't very object-level or concrete
Just out of curiosity, and maybe it'd help readers with answers, could you share why you are interested in this question?