What Kassandra or Alexios wants is not necessarily imported. Certain events must happen. The simulation will find a way to make it happen. There is a point it needs to reach, and characters who believe they have free will end of doing what is required of them. Origins has some
extensive monologues that go into this theme. One of the most important monologues is:
It seems likely that Kassandra or Alexios having a child is one of those events that they have no control over. The child is necessary for some reason that isn't clear yet. It is what the Isu call a Node. A fixed event that comes to pass eventually, in some form, no matter how much you fight it.
The lack of player choice could be the DLC team being sloppy. In that case, it is a fair criticism for an RPG to get sloppy like this. I personally think it's not great writing for an RPG to force the protagonist character into a relationship. It was really glaring when The Witcher games did it, or tried to do it in TW3's Heart of Stone DLC. Or it could be a deliberate piece of "You didn't actually think you were in control, did you?" This is a core story theme that appears in both Far Cry and Assassin's Creed. And this is the kicker, I think. Ubisoft aren't like Bioware. Their idea of narrative choice once involved, in Far Cry 2's case, letting players shoot themselves in the head or blow themselves up. There was no option where you didn't kill yourself. (And people thought Bioware were being rude with Mass Effect 3.)
Desmond has a
son. A lot of people don't know this because he was never directly featured in any of the games. I kinda wonder if people would have minded as much if Alexios/Kassandra's child was never mentioned in Odyssey and instead popped up in later media, Desmond-style. "Oh, hai, BTW Kassandra has a daughter. She's super important to the plot. And she's 100% canon."
I feel like there's also a bit of a clash because Ubisoft do seem to consider certain things to be canon. Alexios is not canon. Kassandra is also not a blank slate. Bioware protagonists are very blank slate-y. Kassandra has a predefined history, predefined beliefs -- this is evident when she makes remarks about politics and philosophy when you're wandering around the game world. For example, someone who is a believer in the healing power of prayer might dislike that Kassandra trusts medicine over prayer and says as much when you're exploring one of the temples. Kassandra is her own person. Players, via Layla, can nudge her, manipulate her, but they are not the only forces manipulating her. Players are not really in control of her past, and they're not in control of her future. The ending of AC: Odyssey has Kassandra performing actions of her own volition regardless of how the player might feel about them. What they are in control of is the stuff that happens in the middle.
Ubisoft's biggest mistake was talking about romances/relationships in interviews and claiming that you would never be forced into a relationship. One could argue this only applies to the main game, but story DLC would have been planned out long before the game released. There's no way they didn't see this problem coming in some aspect.