This is what I meant when I said you're asking them to write another story and character. The abusive nature of the relationship is integral to the narrative. They're not going to make changes that totally undercut the story they've written. What you're asking for is a dark fantasy romance akin to Spike and Drusilla and although the illusion of this is what Astarion is using to entice the player, because he believes it's what they want, the dialogue is presented the way it is to reflect the true nature of the situation.
You don't view the relationship as transactional, but the character you're speaking to does and they want to make that clear because it's an important part of his story. That being said, you do have options to tell him you did it because you wanted what was best for him. It's literally the first dialogue option available. You can also tell him you wanted him to be free, but it seems now he never will be. The first part reflects the player's intention and the second reflects the narrative outcome. I understand you can play a character who doesn't believe that, but they're trying to balance between the freedom to roleplay and story integrity.
If you want the option to miss out on the final scene of his quest if he doesn't ascend, that's fine I guess? But most people aren't going to get all the way through a story and decide they don't want to see the conclusion. They'd have to write, act, and record dialogue for it and for the follow up conversation that almost no one is going to see. It's a waste of time and resources. Whereas a lot of players will choose to end the relationship rather than become his slave, so they had to account for that.
Will also add that while you can't break up with him in the graveyard scene, you can be a jerk. You can be neutral. You're not forced to commit to anything or sleep with him. And you can breakup with him immediately afterwards, up to the very last scene of the game. But again, the scene is more than a conclusion to the romance. It's a potential conclusion to the character's story and they want it to play out in a way that does justice to it because they care about the quality of the writing.
