No one is asking for the removal of the "Karlach dies" ending, because as you said, there is a point where she accepts it.
If you have 17.000 endings as Larian is saying, there should be an ending where Karlach actually survives without being enslaved once again. Arguing the "go to Avernus" ending is a good one is just not true as she is actually returning back to hell, no matter if Tav or Wyll is with her, she is in hell.
So even her "best" ending, is still worse than most of the "bad" endings for other companions. There are so many options to save her and they are right in front of your nose for most of the game and yet all that is done with it is "you think this story is true? im sorry, it's all a lie"
Her ending as MC, when she returns to Avernus, makes it even worse.
The narrator explains how much you enjoy your companions and you love seeing them cheerful and happy "but this is not your path, you have to go to hell alone". Yes, Karlach is hopeful and says "for now" but still, the ending is bad
So let her become a squid, let her die, let her return to Avernus or LET HER LIVE!!! god damn it.
Yeah, I think everyone should be able to look at each Origin's ending(s) and think they all have their place.
For example, Shadowheart fulfilling what Shar wants for her and becoming a Dark Justiciar fulfilling her believed destiny, or Shadowheart rejecting Shar and getting to restore her lost past and find faith in Selune or otherwise. Both have their place, and neither is bad. One is perhaps "Evil" but some players might want to play an evil game. Same goes for the difference between Astarion ascending and Astarion being free. Neither is a BAD ending, they're well written sides of the same coin that have their place. When the solutions for Karlach are so clearly there in the game, but unobtainable by no fault of the player, resulting in endings ranging from bad to worse, even if returning to Avernus is meant to be "Good", we have no evidence or affirmation to back this up. Nothing post ending that suggests her situation is any better than before the game's story, except that Tav or Wyll might be there too, but to what end? Show us. Let her live, but also give her other endings purpose and place.