What I really don't understand is why you think it absolutely, 100% MUST be one or the other... Why can't it be all of them? Why can't all of the endings, both existing and desired, be available as options?
We're essentially playing a Choose-Your-Own-Adventure book... If you feel that Karlach works better by her
going to the Hells,
then by all means, play the game as such and more power to you!
But why must we, the ones who would genuinely prefer the, as you've put it, "Build-a-Bear, live happily ever after" alternative be forced into choosing the exact opposite of such? How does the existence of that option detract in any way from how you choose to guide your particular story through the game? By your same logic, I'd say an argument can be made that Minthara's vanishing from the story after Act II is just the authors' artistic vision, yet I think we can all agree that it would be a dumb thing to argue.
In a game which, as others have pointed out time and again, allows literally each and every other character said "Build-a-Bear" ending as a choice, the absence of such a choice for just one of the characters (and, I might add, the most CLEARLY Good-aligned of them all) cannot be defended as artistic vision, but is damnably just sloppy writing. All the more so when considering that this isn't a linear story and it was never meant to be one.
Hells, if I wanted to watch everyone die at the end, I'd just stick to 40k. There's plenty grimdark in there, we really don't need D&D to be a Snyder flick...
That's my problem. It's not the fact that the bleak, grim and depressing endings exist. It's that it's ALL THAT THERE IS. And also, Larian already made an entire fucking gamemode JUST for you that you can activate in the character creation window if you want to be a fucking murder-hobo. But we still get nothing. Even this "fix" is just that, nothing. People like Raz really be out here eating a whole ass five-course banquet while we're begging for crumbs, and then they have the audacity to tell us that we're greedy and that we don't even deserve our crumbs.