Frankly, I think it's valid to not want to play a "story generator" that draws that arbitrary line.

But the arbitrary line has always existed, and always will exist no matter how many new features they add to the game. Kids weren't in the game for years and the game played just fine because it was balanced around kids not existing. This was a pretty widely accepted piece of "lore" about the game world that bothered almost nobody. It was only once children were announced, that suddenly people had this desire to mutilate them or else the devs are somehow "setting arbitrary lines."

Also, let's take your point to the logical extreme. Should the game feature rape/sexual assault towards adults or children just because "nothing is off limits"? Or what about necrophilia & bestiality? Or could you see how there's potentially a line where it becomes nothing more than a tasteless appeal to edgelords?

Have you ever wondered why subreddits that tout the motto of "nothing is off limits" inevitably trend towards racism, sexism and homophobia?

But I'm absolutely advocating for people to still be seen as people even if they engage with darker fictions than you, personally, can tolerate.

I'm not suggesting this. We're all playing the same game. We've all RP'ed an intentionally evil colony for fun. Even though the whole warcrimes meme is overdone, it's obviously self-aware humor about how horrifically bad their colony is. Someone threatening to boycott Biotech if they can't enslave children doesn't exactly have the same level of self-awareness.

You say you're not trying to make an issue of this because you think it will have RL consequences, but you immediately turn around and the first thing you compare it to is something that has awful IRL consequences: harassing real people.

I say that I don't think RimWorld causes anything because it's not obviously not real. But I do think games & their community can attract particularly nasty people. Do you think it's a coincidence that Hearts of Iron IV happens to attract a lot of WWII "history buffs" who happen to frequent a particular 4chan board and happen to subscribe to a particular ideology?

The comparison is valid, because it's another scenario where gamers are fixating on the ability to do something horrible without the awareness of how ridiculous they sound. Yes, in RimWorld it doesn't have any real world consequences but does that really matter to the point? I'm not talking about the cause of these things, I'm talking about the fact that wasting your time on the conversation in the first place shows where your priorities lie.

Does it "concern" you that an author went out of his way to write that on purpose, or that a cast of TV actors and staff went out of their way to visually portray it in a tense, emotional manner? That, for example, someone on the sound team had to actually ask the question "I wonder whether Bran's back would make a higher-pitched crack sound because of the smaller bones?"

No it doesn't concern me, and this isn't a more appropriate comparison at all.

Again, the concern isn't with the feature being in RimWorld (or in the case of GoT, the character dying). It's not that a game or piece of media in general can never feature violence towards a child or else it's too far. It's that the community was so aggressively focused on the ability to be violent towards children.

Here's a question for you: Lets assume the devs added children and made them invulnerable, but in such a way that the gameplay is still well-balanced. Would you be upset that you can't harm the children, even if the game plays fine? If yes, why? What about children existing in the game but not being killable upsets you? What does the game benefit from crossing that line if the devs chose not to?