Even during my first playthrough, there were a lot of signs of what the ascension ritual does and what can be expected after. There is roleplaying and there is ignoring information on purpose.
Ignoring information on purpose is part of roleplaying, though, otherwise you are meta-gaming. The only information we get about the ritual itself is what Raphael tells us and what we see under the palace on our way to Cazador. But no matter what, the characters don't get enough information to know that Astarion will make a total personality flip after the ritual and become his worst self in an instant. Some more morally inclined characters might, more evil, selfish, oblivious or naive characters might not (or might not care). I guess we just have to agree to disagree as we seem to see the act of roleplaying quite differently.
I read through the entire dialogue tree for this conversation, and where would a second neutral option lead? How could the dialogue tree and later scenes look like?
It is not supposed to lead anywhere new, it is just flavour to let us roleplay our character with a different intention than what the writer railroads us into. It should sit in the same tree as the "I want your body" line. Just a "I don't want anything. I'm just glad you got what you wanted (alt. I'm just glad you're finally safe)." to which Astarion can reply in his best condescending manner: "You're just precious, aren't you? But I really want to do something for you. Don't you want the gift of eternity? One more bite and we could be together forever." Roleplay flavoured, narrative upheld.
