I think about this so often because it's so deeply fucked up.
Like maybe, possibly, there is a universe where JFM and YZY actually did get married because they fell in love or had some passionate affair or something. But that was never going to be this universe, because in this universe there was a political match arranged for them. Usually in fiction, if two characters are compatible then having a match arranged for them just kind of expedites things. But here we see that it's cast a pall on their entire relationship. JFM probably didn't want to marry YZY at first in fact, and YZY knows that, and every subsequent interaction in their marriage is tainted by this knowledge that they are together because it's politics, which means that all the resentment from their early drama lingers and sticks and never gets addressed both because JFM is avoidant and YZY is bitter, but also because there can be no assurance that anything between them isn't just about the path of least resistance. JFM does something for YZY as her husband? Well he's just seeing to his obligations, he's just trying to appease her temper, of course he's treating her like his wife they're married they've had children and the two other people he might have been into are both dead. YZY does something for JFM? Same deal, like at this point they don't have any sensible alternatives, they are stuck together but rather than resolving their differences, it exacerbates it.
This trickles down to their relationships with their children. Jiang Yanli is the failure, but she's a tolerable failure because she's a girl and sexism in their culture wouldn't have let her amounted to much more than a baby factory in the long run anyway. She gets to be her mother's tool in cementing an alliance with Madam Jin, combining their bloodlines, and mirroring the exact same situation as her parents own marriage. JFM meanwhile sees that this is a mirror of their situation and opposes it on those grounds, but this just offends YZY and seems to reiterate that he regrets being "forced" to marry her.
Then there's Jiang Cheng, the ostensible living POINT of all this drama. The male heir who is capable of cultivating to the standards expected of a sect leader. The whole reason YZY and JFM were set up was not only to politically cement the Yu clan's loyalty to the Jiang, but also to try and genetically combine their cultivation potential and get a prodigy out of the deal. That's how it "should" work according to the reasoning behind all the bloodline supremacy in their culture, their kids should be Strong Cultivators. But what did they actually get? A girl dud and an heir who is just good-ish, who actually if anything inherits some of the worse superficial traits of his parents between his mother's temperament and his father's communication skills.
Meanwhile there ARE actual widely recognized cultivation prodigies in the same generation bracket. Among them is, of course, Wei Wuxian.
The thing is, for Jiang Fengmian, Wei Wuxian probably is a way to repair the past but just not in the sense that most people think. Wei Changze left the Jiang to become a wandering cultivator with Cangse Sanren. We don't know the details, but for JFM to be so caught up on that that it's widely known he is even years later, one can imagine that he actually didn't take his buddy leaving very well. He's just not all fire and brimstone about it the way Jiang Cheng later is about Wei Wuxian, because his personality is totally different. But still, we get confirmation when he assigns WWX as JC's permanent bodyguard that this is in fact what he's after here. He doesn't want to replace his son with the child he maybe he could have had with someone else, he wants to get Jiang Cheng the servant that Wei Changze was supposed to be. The eternal loyal righthand man. The carefully groomed lieutenant, exactly like Madam Yu's own personally trained handmaidens who die beside her, or a Jiang equivalent to Wen Zhuliu.
Which is deeply fucked up! And I don't just mean that it's fucked up for Wei Wuxian, although first and perhaps foremost it is, but it's fucked up for Jiang Cheng and Jiang Yanli too. Like there's often discourse about whether or not the Yunmeng trio really do see each other as siblings, but the thing is regardless of how they actually think about it in the privacy of their own minds, they were raised by the same people in the same house. Wei Wuxian didn't have any family that wasn't them, and they grew alongside him for much of their childhoods.
So like, regardless of the societal concepts surrounding it, functionally they were siblings, they were just siblings being subjected to perverse and contradictory treatment at every turn. Wei Wuxian simultaneously being propped up as this "favorite" golden child by word-of-mouth while also being devalued as a servant who's not really part of the family at all. Jiang Yanli the treasured and beloved daughter who is also an abysmal disappointment and walking embarrassment. Jiang Cheng the most important male heir who represents every social pressure that ruined his parents' happiness, like everything happened just so he could exist, but instead of being Great he's just a wet sack full of furious cats.
And they all know their own shitty hand! Wei Wuxian knows that even Jiang Fengmian doesn't see him as a son! Jiang Yanli knows she's weak and disappointing and that her main utility is as a marriage alliance contract! Jiang Cheng knows he's not the ideal heir, that he is failing at just embodying a lot of stuff he was ostensibly born to be. So Wei Wuxian attempts to become a good martyr and Jiang Yanli tries to fall in love with her arranged match and Jiang Cheng tries to put his sect before everything else, and this gets two of them killed and leaves Jiang Cheng standing in the hollow gilded wreckage of society's lies with nothing to cling to but an already-permanently-tarnished image that he keeps trying to wash clean with blood.
