I think Niina needed a challenge, putting her in the group with the highest concentration of top singers in H!P should be good training and motivation for her. She is still young and could carry the group with Mifu even 10 years down the line. Both Ruru and Rei will probably be gone within the next 2 years based on their ages + tenure in H!P. I'm glad that they're making moves for her instead of just letting her sit around as an overqualified kenshuusei.
I don't know if the tradition changes, but I bet that Yume, Riai and Eba-chan will be given tons of lines first. It may take 4 - 5 years in order for Niina to get more lines. If she was put in Angerme, things would be different (Yuki and Hana have been promoted almost since they debuted)
