Aqua could use Akane but I wonder what value she has as an idol and if she's not just dead as an artist.
Ruby was a hard working talent unable to get to higher levels, so it was a good choice to get her because of her potential.
Akane is just a hard working, straight laced nice girl, we understand through flashbacks she has the passion and probably is good if working hard, but that might not be enough.
At least from what I got from watching that episode.