And how in the world she's supposed to be fine with her (presumably) lover selling their soul to the same monsters that enslaved her? She'd literally rather die than stay in the House of Hope, Tav selling themselves out would be like a knife to the back for her.
Oh but sending her back to Avernus, something she despises, is fine? You know, an ending that exists in game. Even before the 'extended scene'. What if Tav doesn't want Karlach to die or go back to Avernus? We are talking about a game where we can have solutions for every single companion. We can either kill them, doom them to their past (Astarion ascending, basically doing what Cazador did even if he likes it, Shadowheart can remain a Shar slave, Wyll can remain as a slave to Mizora and Zariel....) or we can prevent all of those.
We can do literallly nothing for Karlach. And it's off for a game like BG, a game that already provides different outcomes for your companions, minus Karlach. Good, bad, bittersweet. Karlach's endings are all bad.
Not sure why you are against all this, you'll still get your depressing ending anyway. It won't destroy your narrative.
I wouldn't call it fine because in my opinion it shouldn't be an option at all. Karlach's story is about accepting that your time is running out, her character is all about tragedy and living your last days on your rules, sudden magical fix would simply be disrespectful towards the narrative. Same goes for the Avernus option, it is disrespectful towards the character that made peace with her fate and decided die in Faehrun on her own terms, but I also understand that Larian wanted something a little hopeful for her. As for other origin characters, she isn't some special case of having only bad endings, Wyll can lose his father or become slave to a demon like Karlach, Astarion can either become an abomination or live in the Underdark for the rest of his life and Shadowheart can either kill her parents or live in agony for the rest of her life. I don't know why you think Karlach was treated unfairly when it's clearly not the case
Here is the thing though: Wyll already was a slave to a demon. He did sell himself to Mizora in order to save the city, after all. He gets to either take his freedom and risk his father, or continue serving Mizora. And I say "risk his father", because after all you can save him. Sure, Mizora said she wouldn't stop, but at least you have a fighting chance.
As for Astarion, being forced to live in the underdark is a bit different than being forced to go back to your slaver. From what I recall, he didn't even hate the place and called it beautiful - sure, he loved being able to walk in the sun again, but it's still vastly different from having to choose death vs going back to the place that carries a lot of trauma for you.
With Shadowheart it's also more of a free choice - her parents want to set her free, and she can accept that, or she can accept the pain that she already dealt with.
None of these are perfect fairytale endings. But you know what all of these have in common? You tried, you explored the options you were presented with, you looked at the options the game gave you, both in mentions and in mechanics, and used them. With Karlach, that's not the case. It's giving up before trying. You watch the situation unfold and are forced to just stand by and do nothing, when you clearly could. We don't try to do anything, except go to an apprentice smith and hand him two pieces of infernal iron. There is not even a follow up with the Gondians - you free them and then you can't even inquire about the engine, even though they clearly are the experts? Mind you, they are just one avenue out of many that they show us before refusing to allow us to explore it. It's like having a broken leg and watching it get infected and going "well, thats it, I'm going to die" instead of at least trying to let a doctor take a look at it.
That is the main issue with the storyline as it is right now. It's one thing to have some tragedy and a price to be paid, another to railroad the player into it and forcing inaction to the point that it honestly feels like they and the companions do not care, as opposed to trying and failing.
