I became a Berryz fan after Maiha left, but since becoming a fan I have listened to pretty much all their old stuff, and seen all their old videos, and in doing so, I never really noticed Maiha. Her voice was nice but, she never really stood out (to me), and I personally think Berryz wern't damaged by her graduation. But of course like I said, I was never a big fan so, maybe a Maiha fan would think differently.
The sales could be better, but there is a good chance they will be soon. The thing I don't understand is people saying wota are keeping them going. But surely wota are just fans? Don't fans keep every group going? Aren't we all fans? Maybe just not as huge of fans as the wota.
Very few groups, and mainly girl groups, and especially child groups, don't last very long. Berryz still have a few years until I (personally) feel they would be ready to start solo careers, and even though it would be horrible to see them break up, I doubt we will never hear from them again.
Risako, Miyabi, Momoko and maybe Yurina could have a good solo career ahead of them. Chinami's voice is getting better and better, so I doubt she'll stop singing. Saki I'm not sure about, I think she's great though, and Maasa will more than likely grow even more beautiful, and maybe sell some PB's or something.
I don't really think they need generations, the current Berryz will always be Berryz, any new members will just look out of place. Plus they are all still pretty young, and are only improving. Better promotion will gain success, not new girls.
All in my opinion of course