Tav isn't meant to be a character we imbue with deep feelings and a unique internal life from playthrough to playthrough, they're meant to be a vehicle we drive through the game to see stuff with.
Well if this theory is true, then they really need to reconsider their target audiences.
I believe possibly Astarion was designed to attract mainly female players (I could be wrong), and they expect that female players don’t imbue with deep feelings? Who did the marketing survey in Larian seriously lol
I doubt that was their thought process. I just figure that they expected players to care about the companions themselves and to care about seeing the companions have a relationship with Tav, but that Tav is there to serve as a someone they can use to steer the companions to show the types of relationship the player wants to see. Tav is basically meant to be the lever used to steer and manipulate the world. In the case of Ascended Astarion, the new kissing is there to show another, more clear and more obviously negative outcome. You've created a story where your Tav is trapped under a manipulative abuser. I suspect that to Larian, Tav themselves isn't meant to have their own thoughts and feelings. Even if you headcanon that your Tav was all on board to be an evil power couple, that doesn't really matter because Tav doesn't have opinions, they're a limited mouthpiece for the player and then a sounding board to see the other characters who actually are characters react and express themselves.