A good ending where a character lives on should feel like a beginning, and this one doesn't. So they should expand on the "go to Avernus" option. At least show us that this is, in fact, a beginning of a new story. Show us a scene with them stepping out of a portal, taking up their new lives in this inhospitable place, but well, Karlach has always been good at survival, and now there are two swords instead of one, so that's fine. Maybe give a hint that they might find a permanent solution to her problem here where they make the stuff her engine is made of. As it is, with things just stopping, it is not satisfactory.
Meanwhile, I also think that "Karlach dies here" is appropriate as an ending. Karlach is a woman who lives in the present. Death comes for us all so why worry about it until it actually does. It is an attitude that fits her and calls back to the old heroic fantasy stories, expressed so vividly by Valeria in the old Conan film, calling out "do you want to live forever" as she jumps from a tower into a pool of water which may or may not be deep enough to survive the jump. It is a reminder that all good things come to an end, and it is telegraphed early enough to the player, so you can adapt to its possible inevitability.
