>>534576816
MC (Harmony) is required in her team because MC and Firefly is being shipped heavily in the story back then. But MC's new form (Remembrance) from the current story is needed by a more powerful team. So she is now useless.
Just imagine if Cartehyia's kit revolves around Aero Erosion but can't actually apply it, then Rover (Aero) is the only character in the game who can apply that effect so they fit perfectly as a team. But then New Federation comes and Rover (Electro) is needed by the new meta. So now Cartethyia is stuck with a kit that she can't use.