<...> What Karlach doesn't want to return to in Avernus is the loneliness. Devils make for poor company for someone who yearns true love and companionship, someone with the wants and desires of mortals. She wouldn't have been abused or tormented, her position in Avernus would have been concrete and comfortable for as long as she could perform in the Blood War, which was easy for her. She is terrified of going back to that unthathomable loneliness, but she wouldn't have that with Tav by her side, which is why her reluctance to go back with Tav is so confusing. <...>
Well, I would argue that in the long run after ten years it doesn't really matter if the reason she hated Avernus was the loneliness alone and if going back with Tav fixes that issue. That's still ten years of enough negative feelings and connotations to drown out (or at the very least clearly not being enough to outweigh) whatever was tolerable or even occasionally positive. You don't scream to high heavens and break everything and cry due to being free from a place, as well as adamantly refuse to go back and be willing to rather choose death (at least at first) unless the time there REALLY got under your skin. Even if the main reason it sucked so much is negated via Tav, the trauma doesn't go anywhere. It still sucked, and everything about will still remind you of the way it felt before. Then there's also the feeling that you finally tried to make a choice to leave on your own terms and in the end that choice was denied to you. Missing home and the world outside also never really goes away, I wager.
So while yes, it's good to remember that DnD's Hell and particularly Avernus is not a stereotypical pit of misery, I don't think Karlach's issues with it - and with going back there - are negated by the fact Tav's going with her and it may be better now. That sure helps, but that pent up years-in-the-making repulsion doesn't just go anywhere. Even if they are given good positions and conditions and stay together, going back at all is gotta sting.
(Not to mention that on behalf of the character narrative that would kind of feel... patronizing, I dunno? Like, 'See, it wasn't that bad here, happy to see that you finished having your little tantrum and returned where you belong'. Not the worst ending, sure, but frustrating nevertheless.)
Disclaimer: Still haven't finished the game, so I may be missing some dialogue that might indicate she's coming around, but going off of others' posts it doesn't seem like there's any lead up like that at all. Which just makes going back feel like defeat in the end, ultimately. 'I tried so hard and got so far but in the end it doesn't even matter' (c) type a thing. So yeah, best of the bad endings? Sure! But the best, period? That's cruel.
Yeah I get all of that and you're right, I'm not saying she should live in Avernus or return to Zariel if there are other options, just as a possible outcome and the only outcome we have right now in this miasma of despair that are her endings that could end with a positive resolution down the line. Tav and Karlach could plot against Zariel with another powerful Devil to overthrow her in return for Karlach's heart and freedom, defeat her themselves without ever entering servitude by playing the Game of Baatur with some of the powerful but imprisoned Devils in Avernus, which is exactly what happened in Descent. With how powerful we're becoming by the end of the game, Tav could even enter a pact with Asmodeus to go over Zariel's head. There's a billion possible resolutions to returning to Avernus that end with freedom back in Faerun, and by the Hells do I want to fight some Pit Fiends! Especially after their awful representation in BG1&2.
I really hope nobody thinks I want her to return to Zariel for eternity or some sh*t, I love the lore of Baatur and can see the potential in returning there with her, and it's one of the paths Larian could take that leads to the resolution she deserves. Could even enter an agreement/bargain with Withers to return her from the City of the Dead with true resurrection. Something. Especially after Gale
has an almost identical story with higher stakes that he escapes from
