I don't think framing it as Larian getting lazy or leaving her story unfinished is productive (or accurate!) The issue with Karlach's storyline is that she's tied to a lot of thematic points that no longer exist. Karlach outright tells you that she's going to live until she burns up, that she wants to spend it as happy as possible. That this is what ends up happening to her isn't a bad ending for her. It's only a bad ending because every other character can dodge their fate. It's most obvious with Gale, where Larian made it comically easy for him to just keep dodging his flaws and consequences. The problem of Karlach isn't that her ending is lazy or unfinished, it's that it feels tonally and structurally dissonant for one character to be given a bittersweet ending about accepting their situation and making peace with it when no other character has to. Like I said, I think she's a remnant of the earlier narrative. The story changed, her ending didn't.

Apparently, Karlach can also
volunteer to turn into a Mind Flayer when someone must do so near the ending, and figures she's happy existing as a Mind Flayer.
But that option didn't come up for me when she was present for the conversation.