Alternate title: learning from fictional evidence. I've seen echoes of this idea elsewhere but couldn't find a description that suits me.
My main idea is: you can update from your observed reaction to fiction and/or counterfactuals.
The fallacy of generalizing from fictional evidence happens when you treat events having happened in fiction, following the rules of good writing rather than verisimilitude, as observations of reality. The facts may be wrong but, if you suspend your disbelief for a while and get immersed in the story, your emotional reaction will be real.
Compare this to counterfactual reasoning or forecasting scenarios. You explicitly build alternate versions of reality. You will actually experience only one. (Others may belong to other Everett branches if they're consistent with your past, but not yours!)
Here as well, you will be considering your reaction to those alternate version. Is this possible future worth avoiding? Worth fighting for? Ideally, this evaluation should be separate from how much probability mass you assign to this particular scenario.
In his original essay, Yudkowsky critiques the jump from "sci-fi displays a compelling narrative" to "sci-fi portrays realistic/probable scenarios". Indeed there's a jump which shouldn't be made between the cherry-picked manufactured whatever scenario, and careful forecasting.
Not about what is actually happening (or has happened or would happen), but about you. The fact that you could believe a particular scenario says something about your ability to be confused by fiction vs. reality. If, when breaking out of immersion, not longer suspending disbelief, belief remains, it should give you pause.
More importantly, if the fictional narrative gets dismissed, the emotional impact shouldn't be. I can't find the original quote: "nonfiction is about conveying fact, fiction is about conveying experience". Your feelings are valid. One point of rationalist fiction is to immerse you in situations where characters solve problems, reason carefully, work with their emotions instead of against them.
Personal example. Consider this Doctor Who clip, where a struggling Vincent van Gogh is taken to 2010 to receive high praise from a museum curator. It never happened. Yet, the acting, the tragedy, inspires me and gives me motivation. Same goes for HPMoR, which definitely never happened and still moved me.
This is "fictional evidence" not by the plot being real, but by the evidence of a reaction to fiction being real.
So what kind of updates can you make on it? It helps build, for me, a better model of my own motivation.
That my judgment about some art or page of mine being worthless should be anchored by other people's feelings rather than my own (occasional) despair. That there is strength to be found in solving problems collectively. That it's okay to pause and think, and take time for yourself.
All of this and more, I learned from fiction before experiencing it myself. Same goes from all the existential risk scenarios that didn't happen, and hopefully never will: my reaction to them still informs my decisions today.
This is distinct from Wei Dai's fictional insight, where fiction exposes you to a particular idea/hypothesis/thought-pattern you hadn't considered before and expands your mindset. Here, I focused on how your relationship with not-our-universe can inform your relationship with reality, by virtue of being experienced by the same brain.
Thanks to Adam Shimi for his feedback on the draft of this post.