Karlach suffers from a lack of content in general. Again, you might be reading too much into it.
Larian would have to ignore an ENORMOUS amount of lore and current items and people in the game to double down.
But to what purpose do they need to ignore the numerous hints of a good ending for Karlach in Act 3? Do they hate her and want her to die or go back to hell? Which she just emerged from being emotionally unstable?
Because changing her ending takes time.
It's not just add one quest. You have to add it in the right place to make sure her emotional beats still hit. You also have to completely change her epilogues which can cause issues with other epilogues firing.
