it would reduce these two characters and their relationship to "just another romance" when they have other sides to them that are, in my opinion, much more interesting to explore.

THIS. I really like Anna and I really, really like her friendship/sibling relationship with Castiel, but I would just be bleurgh about a romance (and a contributing factor to that is how much something that codes as incestuous being treated as a loving pairing would squick me). I'm also drawn to platonic m/f relationships, be it familial or just friendship, and I think it's in part because they're so goddamn rare.