I might have missed it in your post, but descendants of humans encountering a grabby alien civilization is itself an (agential) s-risk. If they are optimizing for spread and unaligned ethically with us, then we will be in the way, and they will have no moral qualms with using morally atrocious tactics, including spreading torture on an astronomical scale to threaten our values to get access to more space and resources, or we may be at war with them. If our descendants are also motivated to expand, and we encounter grabby aliens, how long would conflict between us go on for?
I might have missed it in your post, but descendants of humans encountering a grabby alien civilization is itself an (agential) s-risk. If they are optimizing for spread and unaligned ethically with us, then we will be in the way, and they will have no moral qualms with using morally atrocious tactics, including spreading torture on an astronomical scale to threaten our values to get access to more space and resources, or we may be at war with them. If our descendants are also motivated to expand, and we encounter grabby aliens, how long would conflict between us go on for?