Though Lan Wangji failed to save Wei Wuxian's life by turning on his own sect to protect him, his interference did one good thing; it allowed Wei Wuxian to be in the position to destroy even half of the Stygian Tiger Seal. If Jin Guangshan had gotten his hands on the whole version, the future would have been very dark indeed. Instead, he ends up having to keep Xue Yang around to try to fix it note While Xue Yang did manage to recreate the other half, Jin Guangshan was already killed off by then. ... something that ultimately leads to Wei Wuxian's return. So though Lan Wangji didn't save Wei Wuxian in the normal way, he paved the way for him to come back in the end.
According to Lan Xichen, after his parents married, the circumstances behind their marriage caused his father to isolate himself and never share a bed with his wife. If so, that raises questions on the possible circumstances behind Lan Xichen and Lan Wangji's births.
Considering the Unreliable Narrator that is Wei Wuxian, it's easy to miss the hints dropped in regards to Yu Ziyuan's Parental Abuse directed towards him; this includes things like him referencing being whipped a lot by Zidian, him brushing off Wen Chao's humiliations and taunts as Yu Ziyuan was worse to him, references to old wounds, references to Jiang Fengmian having to step in so Wei Wuxian would get medical attention, and her venomous words directed not only to him, but to Jiang Cheng as well. Considering how much Wei Wuxian downplays it even as the truth slips into the narration even so, one has to wonder just exactly how bad it was for him...
In the story's timeline, Wei Wuxian spent around three months in the Burial Mounds. One can wonder how he managed to survive for that long in a place where there's likely no food to be found anywhere. While he might have practiced inedia, there's also the more horrifying alternative.
Jiang Cheng: You're farming on a mountain of corpses? Will the things that grow here be edible?
Wei Wuxian: Believe me. When people really are hungry, they'll eat whatever they can.
Upon a complete reading of the story, readers will know that Lan Sizhui is Wen Yuan and the only Wen to survive the massacre that took place in the Burial Mounds... because he was hidden away beforehand. So it's easy (or not) to imagine his fate if that wasn't the case...
The author herself has stated that Jiang Cheng has been blacklisted by the female cultivators' after three disastrous blind dates. This makes it nearly impossible for him to find a wife. While Jin Ling also has Jiang blood by virtue of his mother, Lineage Comes from the Father is applied in the setting of Mo Dao Zu Shi, meaning Jin Ling is a Jin clansman first and foremost. This means that unless Jiang Cheng gets a stroke of luck and meets his fated one or makes some arrangements or self-improvements that would get himself off the blacklist, he's likely to stay single for the rest of his life. And if he's unable to cultivate to immortality, there's the possibility that the Jiang line might die with him.