Jessica is Herbert's creation, but she lives past the events of the first novel. The way her arc plays out is a core driver of the tragedy, and to me, her survival isn't a spoiler—it's the point. She bears the weight of her decisions, raising the son she was told to have, and her whole journey is about facing the monstrous future she helped set in motion.
You don't get to see her much in 'Dune Messiah', which always disappointed me, but her presence is felt in every choice Paul makes. Her story ends, in a way, when his truly begins, which is a more interesting kind of death for a character like her. She fades into the background of the legend, and that's arguably sadder than a physical end.
I remember being relieved she wasn't killed off early, but then realizing how hollow that relief was. Herbert gives her life, but takes away her purpose, and that's the real gut-punch.