I agree with all of this but i think in a way ekko and mel WERE kinda treated like after thoughts by the writers themselves compared to the main 5. Well, ekko at least, altho i have a few problems with mel’s s2 arc being too convenient. They definitely both deserved better.
Just wanna say that I think Ekko and Mel slander makes sense when you understand that 85% of the Arcane fandom views both of them as characters less worthy of fandom attention.
In their eyes, they are afterthoughts to more interesting relationships, so when someone in the 15% dares to give them anything outside of dutiful lamentations ("Mel and Ekko deserved better!"), they can't help but feel those analysis would be better suited for a character they *actually* like and so they lash out.
Fans of both characters have to constantly argue that these characters have loved and have been loved. They want Mel "independent" because she doesn't "need love" and they want Ekko focused on The Tree and the unnamed Firelights because they want it to be easier to carve both of them out of the interpersonal narratives that attaches fandoms to characters in the first place.
They want Mel and Ekko to "save the day" because they don't want their roles in the story to interfere with other characters that they like. Better for them to beat the Bad Guys™ than to be given more material that would validate their personal connections with other characters in ways that the fandom would prefer to ignore.
And the fact that Ekko and Mel are completely different characters who have never once interacted with each other and yet somehow both of them are being treated the same way with how the fandom constantly diminishes their relationships with the rest of the main cast leads me to believe that it does have something to do with the One Trait they do share.