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.
