They spent 200 years figuring out time travel to change the past. That's 200 years they've already survived. People with next to nothing gave what little they have to help them in their work. If you've read the story, you'll know it ends with Midgardsormr waking up, being inspired and helping them rebuild. It's a story of hope, not one of a doomed disposable world. The source has survived 7 rejoinings, I believe that timeline will survive the 8th, because my hope actually is everlasting even in the darkest of times. The First that we saved was impacted way worse by Light and they will rebuild. There is no reason to believe the Source in that timeline won't also rebuild the same way it did the seven times before. They risked their timeline being destroyed, but it wasn't, and last we hear of them, they are building towards a better future.
Perhaps the mistake in writing wasn't in not killing them off completely, but in perpetuating the idea there are people unworthy of life because they will die, ignoring everyone will die. Even the races who died off entirely, their souls remained to create new life and thanks to dynamis, even their languages, cultures and histories survived in Ultima Thule. If longevity is the metric by which someone's worthiness to live is measured, then the Ascians were right. "The ephemeral lives you exalt are pale imitations, utterly devoid of meaning." This is the viewpoint you're holding, but with civilizations rather than individuals, but it's wrong by both metrics.
My theory on how this happened was they were setting Venat up to be evil knowing we'd be killing both Zodiak and Hydaelyn, but that idea got vetoed by a higher up, so they switched up her tone without changing her actions. Cause I can't imagine from the onset they were aiming for her to be good and started with "then she sunders everyone with the full knowledge of the hell she'll be putting them through."
Well, as long as we don't do a colonialism. *begs* Please, please, please don't have us go there and colonize these people. I don't want to spend the next three years trying to explain why being a colonizer is a bad thing.