Yea karlachs quest line only revolving around her engine needing to be fixed was a mistake, especially since it really only is a fetch quest. Completely reworking her questline to be something different would be a massive undertaking though, for one they'd have to remove/add so much more dialogue not only for karlach but the companions as well. Theyd have to bring back so many new and old actors, do all the voice acting AND mocapping, plan a new script for her questline, new endings - its a lot of work for just fixing one character, even if the players have been super vocal about her questline.

Larian could add some avernus ''dungeon'' area where we can explore karlachs past and find a cure(same way the other companions get an area to explore their stories, like the creshe for laezel, cazadors mansion for astarion etc) and have it take place after the confrontation with gortash. That way the questline would be exactly the same as now and larian wouldnt have to change anything up until after gortashs death, the only new content would be avernus itself and a new ending. But thats probably also unlikely to happen since that would also take a lot of work. I guess they could make an avernus adventure dlc although it would feel pretty shady to have karlachs quest locked behind paid content lol