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.