tl;dr: people wanting the option to say 'I don't need anything from you' is not affecting Astarion's storyline or the narrative in anyway, it's a neutral statement that doesn't express happiness, and insisting it does either is kinda weird.
RP is always restricted to >10 options. On several occasions, the player is forced to make serious decisions without a real neutral choice (this is true not just for romance or Astarion dialogues, but for all origin characters)
This post-ascension conversation doesn’t exist in a vacuum either, there are several previous conversations, that ask the player to seriously consider what the ascension ritual is, why Astarion wants to do it, and how vampires behave. By this time the player should be aware that ascension is his “bad” ending and have a reason for going through with it.
There isn’t a “I don’t want anything from you” option, because that option is convincing him not to ascend.
The player character does not know what will happen to Astarion when he does the ritual. For all they know he'd be the same person they fell in love with/befriended, just more powerful (there's even a dialogue option that states exactly that). What we as players know and what our characters know are two wholly different things.
I understad that not everyone is into heavy roleplaying when playing a game like this and a lot of people self-insert (I usually don't), but for us roleplayers a railroading dialogue like this can feel quite frustrating. Especially since it was working out fine up until that pivotal moment.
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.
