Seems like they already are stabilizing at 30k. With the exception of S/mileage who is now selling less than they were prior to 2012 everyone has reached a new normal in terms of sales.
Morning Musume - 100k
C-ute - 50k
Berryz - 30k
I think before that it was 50k - 20k - 20k Though I know Berryz was at 30k before when they were doing Inazuma 11.