I don't have a problem with there not being a happy ending, or with the game "railroading" you into seeing Avernus as the bad ending. One of the things BG3 does extremely well is challenge the way most people think about video game companions. We're conditioned to treat our companions in ways that would, in real life, often just be enabling shitty behavior and not being a good friend. But in BG3, doing that gets you some pretty shitty people for companions by the end of things: a psycho religious zealot, an arrogant demigod, a douchey turbo-vampire, etc. At the same time, the actual relationship between the player character and the companion characters is refreshingly dynamic, with you being able to influence people and how they think without first needing to meet a certain threshold of "so-and-so liked that" points.

So with all of that said, if the only choices are Avernus or death, I agree with the game as it originally framed it: Avernus is the bad outcome. That's the outcome where you didn't respect your friend's choices, you kept pushing and pushing and pushing--and if you romanced her, you probably guilt-tripped the shit out of her. You don't know how horrific her life was in Avernus, but you can get a sense from her post-Gortash speech. And now you've pressured her right back into that same horror, just to assuage your own feelings. You pressured her into accepting a life of pain and suffering that she did not want. If you did that to someone in real life who had a terminal condition, you'd be a real asshole.

The problem only arises when they give you such an obvious third option, and even hint at one or two other possible ones earlier in the game. And then they have the gall to put the words "we did all we could" in your mouth.