I think the point is you are supposed to feel Noctis sorrow/anger at Ardyn, rather than sadness for Luna. I kinda think people are missing the point of her character insofar as she isn't a "romantic interest" figure so much as a saint who's destiny is knowing martyrdom.
The whole marriage angle struck me more as a symbolic gesture, both in a political and mythic sense, than any sort of love match. Her dutiful devotion is rewarded by being made Queen in the afterlife, which ironically I think subverts the criticism of her usage being overtly sexist. From her character's perspective King Noctis is just as much a "trophy" as she is to him.
To be completely honest, I don't think anyone has a good grasp of what Luna's role in the story is supposed to be. I think Tabata was completely right when he described Luna as heroine that you can't ever quite reach, mostly because she doesn't even feel like a real character as she has pretty much no screentime for someone who is supposedly a major character and the "core" of the story.