The sales forecast is not so much optimistic, considering transitions of recent AKB, SKE and HKT singles. NMB’s sales are not an exception (but might be a little increasing, for an album is twice the price of a single).
However, that’s one of the reason why the office could rather easily try new center formation to prepare the next evolution. There’s nothing to lose. Featuring Miyawaki Sakura in AKB single perhaps comes from the same policy. Maybe SKE single follows.
As for the case of NMB, the accidental event that Milky won the solo debut made things easier.
It’s sorry for Yuuri to lose her seat. But she’s still 14 y.o. and the premium movie of second album tells she's got a certain position comparable to that of Miru, i.e. a center candidate from non-center starting position in a team.
Teenagers outstandingly promoted by NMB office are correspondence high schoolers (such as Fuuko and Nagisa holding additional post, and as Jo Eriko featured in a teleplay). On the other hand, for girls who are full-time high schoolers (such as Yoshida Akari and Shuu), the office keeps room for their commuting.
In a recent mail, Yuuri hints that she’s not going to a full-time high school. Her hometown is a little far from schools which have entertainment courses. Akari and Shuu are students of one of such school so as to balance their activities. So, if it becomes true, full-scale promotion of Yuuri is likely to take place in next year, and it’s time for her to prepare for it and enjoy school life left.