You say that as if they had that power. Niki has as much power as Akira lets her. He's the only one for her and she can never give up on him. That is not the same for him in the least. Niki is aware of this and the only moment where she has put her foot down was when Ami had her back, and for all Niki means to him, Akira would have thrown her out if not for Ami having called her. So she might try (don't think she would), but if Akira doesn't give up on his harem for Ami, he would hardly do so for Niki.
Wakana is more capable of doing something because she can live without Akira. Wakana however knows more about Akira than Niki regarding his relationship with Sekai, and even if she found out and tried "dismantling" his harem, what could she actually do? Have him fired? He clearly doesn't care about that. Arrest him? That wouldn't happen either, and neither would Wakana even try it as a way to help him.
Definitely agree that they'd call him out, but yeah, they have no power there... So far, the one person I see with the highest likelihood of Akira actually listening to regarding this is Maya Prime if she returned Edit: (because, well, he was faithful to her for years. That's the only person he was faithful to, since he was with Sekai at the same time as Niki).