I think people are overlooking the Avernus with Tav option. Knowing Zariel in DnD lore, she would accept them both as Champions, and they could fight the Blood War together. While this seems like a cruel fate for Karlach, from her dialogue we can see that she didn't run from Avernus because she was a 'slave', she ran because she was alone, having her chance at love and connections stolen by being stowed away into a realm of heartless Devils (enforced by her Compassionate approval tag). She had respect and titles (Demonbane), Mazora was jealous of how much Zariel favored Karlach over others, and Karlach admits that she enjoyed fighting demons (further enforced by her Violent approval tag).
It would be very in character for Zariel to forget about Karlach's insubordination should she come back to her willingly, and pretty much guaranteed with Tav offering themselves as another Champion (A base level 12 character with direct patronage from a Lesser Deity is extremely powerful, even more so in the Deity's own realm of influence). It is bitter sweet, and horribly resolved in the actual game with no sliders or post credit scene or quick in-game battle against Demons surrounded by friendly Pit Fiends etc., but Tav and Karlach continuing their lives and romance as Champions of Zariel is not a bad fate by any means, and would cure Karlach of the loneliness that ate at her in Avernus. The main issue with this ending is the execution, the Soprano's style fade-to-black with no shown or implied resolution. For all we know Zariel immediately kills Tav and forces them into a soulcoin to punish Karlach (very unlikely, Zariel puts protecting the multiverse from the Abyss above all else) or they arrive in Avernus and immediately run for their lives being hunted like rabbits. The resolution isn't there, their fates are left to interpretation which in other stories can work, but doesn't work here.
Now I'm not saying that she shouldn't have a more difficult resolution to her quest that sees her live happily in Fayrun alone or with a romanced Tav, she should. Returning to Avernus with Tav should be an outcome from a quest-fail state combined with a successful romance, not the best outcome. However it's the best outcome we get, and knowing DnD lore, their fate would not be a terrible one. They get to have their fairytale romance in the downtime between fighting in the Blood War. We are just not shown this, and Larian failed to resolve the storyline they implemented with poor execution rather than poor ideas.
You know what, thank you for this (and the following posts on the topic), because I indeed didn't have much context on Zariel. Just like others (and you yourself, credit where it's due), I still maintain that it sucks plot-wise and DnD-mechanic-wise that we don't have an option or a choice to struggle for a happier ending for her (especially with opportunities presented and compared to others), so to that end I'll MUCH hope for an addition somewhere down the line, but for so long as we have to work with what we have for now... That's an interesting point to consider. Will make imagining what could happen after much easier. The bitterness of the fact that she absolutely never wanted to go back there and that nothing really progressed for her character except for Tav (and being able to touch people at last, hopefully that stays) is still there, but at least it doesn't sound that bleak.