Except for a couple like Reina and Kaede to name a few, all the girls are pretty much new. To keep them would bring in more older fans who will later become fans of the newer girls.U need SOMETHING to attract the people to the shows. Plus Tsubaki is till a Kenshuusei group without a major debut yet.
she's be a perfect candidate for next gen Country Girls. she could be who they add after Momoko leaves, I mean she's already got a sortof musubi thing going on already.