Tsunku likes that there are lots of new girls. Watching that special interview they did, he mentioned that he had in mind to pick 8-10 girls for 9th generation. He wanted more new than old. Thankfully due to lack of quality, he didn't get that. And that's why we got 10th so quickly. 11th was probably - like the title hinted at - because of the lack of singing quality.
Having that said, Aika's leaving was very sudden and most likely not a part of his master plan. Because of this, I think that either Sayu or Reina will stay for at least 1½ year more. Aika would have been a great leader once the two of them would have left, but as it is she's gone and Mizuki is not ready to be leader yet. To be honest I doubt she'd be ready by next year, too. Which is why I think Sayu will graduate sometime next year and Reina will be leader for a little while before it goes to Mizuki.
Tsunku also mentioned in that interview that he wanted to bring sub-units back to H!P and have them tour etc. Which would be great, as it would help the girls grow and also give Reina time to do her band while being in MM.
MM needs Reina right now and while I would love to see both Sayu and Reina for another few years in the group, I think they need Reina more because of her vocals. She is the best singer aside from Sakura in the group and they need that. No matter how much idols are not "about singing talent" or whatever, they need her.
Tsunku said it in that interview too, that no matter what he don't want the group to drop in quality and lose to other groups whether it be to dancing or singing. And until 9th and 10th become strong enough they need Reina's voice more than Sayu's.
Kanon and Kudou are doing good with filling Risa's lower voice when it comes to holding up the unison singing in songs, but there's no one aside from Sakura who get close to Reina's level right now. Mizuki will eventually, Riho is getting better day by day and Masaki will bloom soon I'm sure. But not within just one year. They'll grow a lot, but not enough.
If I would do this, I'd have Sayumi graduate at the end of next year - announce it in January - and then have Reina be leader a bit before she goes full time with her band sometime in 2014. Leaving Mizuki as the new leader of MM.
Watch them graduate both Sayu and Reina next year though...