That is fairly simple to answer: If you say "this is canon" you say "this is real", Astarion being a vampire is canon. If there are two lines of interpretation that are contradicting, saying one is canon not so subtly implies that the other is wrong, leading us back to the "I am right and you are not" front.
Ok, but I do in fact think those who don’t see the abuse are wrong in their interpretation. So what? Why should it matter to them what I think? They aren't being forced to engage, they are choosing to despite knowing where I stand.
The only opinion I’m trying to change is Larian’s via my feedback and *ideally* the only thing I am interested in discussing is the best way to implement this change.
Because it is a sensitive topic and, as you might have noticed, both sides (or at least some proponents of them) seem to feel somewhat threatened by such statements due to bad experiences. You can convey your sentiments to Larian just eloquently when voicing things as your opinion, preferably with giving reasons where you derived them from, instead of just framing them as canon. It's less offensive, conveys the same message and allows our dear Red Queen to maybe let the sceptre of moderation rest for an evening.