Another thing to consider: what happens to the wota?
Berryz Kobo have enough fans to reliably push between 20.000 and 50.000 physical CD's every release.
DD and MD's excluded.
A certain percentage will quit following idols altogether. That always happens with every graduation of any idol.
In fact, after 10+ years, some of them may be looking forward to 'freedom'.

But the rest?
Will they....
...jump in the familiar arms of °C-ute?
...move over to Juice=Juice, Morning Musume or S/mileage?
...get enamoured with the new Country Musume, Morning Musume 12th gen or a possible new H!P unit?
...start backing Trainees in the hope their bet pays off?
...switch to non-UF-idols (AKB48, Momoiro Clover Z, LinQ, Afilia Saga, Rev.from DVL,...)?
It's going to be interesting to see how this all develops.