Personally, I don't think there's anything wrong with wanting an option for a happier ending for a companion character. If you can't save everyone, that's fine--but usually we get the option of who we DO get to save, as in the Tieflings vs. Minthara example. Not giving us a similar choice for Karlach feels to me like a cop-out. Tragedy for the sake of shock value, instead of as a result of choices.
People like to argue, sometimes, that wanting to play games/read books/etc. with happy endings is either boring or is suggestive of the fortitude of the consumer's character or will, and I find that highly reductive. People have all sorts of reasons for engaging with media, and that's their prerogative. Personally, I'm a big fan of horror and tragedy done well. I simply don't agree that Karlach's was done well, for reasons listed above.
Likewise, I don't think that having a happier ending would cheapen the work done in the sadder endings at all. It's about options and picking the choice that feels right to you, which is the entire spirit and appeal of D&D in the first place. As a DM irl, I admit to sometimes being a little disappointed when my players don't choose a route I really enjoyed writing--but if I insist upon something they don't want, then we aren't playing a game anymore. Then I'm just telling a story.
