So you'd rather have a boring explanation with some magical bullshit that fixes everything instead of great story we already have? Yeah, Gale can be saved by Mystra, but she's a literal goddess of magic so knowing her has its profits, Karlach didn't have that luxury which is why we can make her last moments great after all she suffered. That makes it good from narrative standpoint
Okay, let's take a step back here. I never said that I would like Karlach's story to resolve the same way as Gale's. Frankly, Gale is my least favourite of the origin companions. To me, his story has bad fanfic energy. The Elminster stuff is particularly cringe. In all honesty, I actually think that Karlach's story, as it is now, is better written. Her sad ending is bittersweet and respects the values she's insisted on from the start. Her more hopeful ending lets her re-evaluate that in the context of the relationships she's forged (and is decently open-ended - who knows what she and her companion(s) might achieve together to change things up?).
Nonetheless, I think Kneecap makes a valid point that the story bends over backwards for Gale and seems to put as much effort into boxing Karlach in. I had not considered the potentially problematic parallels that Kneecap highlighted before reading that post. Can't we all learn something from each other's perspectives?