I think an ending where Karlach is doomed could be satisfying and cathartic, but (like others have pointed out) I don't think the writing does enough to convince us that her condition actually is unsolvable.
We talk to only 1 NPC about her heart. We don't even bother to get a second opinion. The Gondians work with more advanced versions of the tech that make Karlach's heart. Neither she, nor the player bother to ask them for help. No one bothers looking into magical solutions.
The writers could probably come up with some sort of reason why these wouldn't work, but they didn't address it at all. As it is, it comes across as the player not trying to solve the issue, or we have to headcanon reasons the solutions wouldn't work. An obvious reason why the magical solutions are not achievable is cost, but that wouldn't make this a story about terminal illness. That would be a story about inaccessible health care. Of course people weren't satisfied.
As it is, I headcanon and metagame to get an ending for Karlach I'm happy with. I trigger Gale's first death before meeting Withers, and use up the Scroll of True Resurrection (ignore our scrolls of Revivify, shush). With the new epilogue, I'm headcanoning she will be cured. I'm happy with it becoming an escapist fantasy tale of teamwork and perseverance.
