i agree with the "seniority"-think. because "almost" all of the leader is the oldest (except Yossie-Ishi leadership. Yossie is the leader and Rikachi is the sub, even yosiie is younger than Rikachi) and more experience than others. Even Mizupon is younger, but she is more experience and senior. even i feel she is more fit to be sub-leader than Harunan (i can feel her leadership, not because she is the oldest (after sayu), but her presence is tell you she is leader-like.) CMIIW
^^Wat. Rika already had plans to graduate once Yaguchi became leader (Rika's graduation announcement was around the same time as Iida), thus Rika never became sub-leader of Morning Musume. All the other sub-leaders didn't had their graduation planned when the were given the responsibility.
I doubt Miki would have became leader if UF knew she would leave MM a month later.
Erika Umeda was the oldest in C-ute, but she didn't want to be the leader, so Maimi became the leader instead. Seniority is the primary factor, but I'm assuming the girl's that is to be leader by default can choose not to accept it.
So? They still did 3 singles, 1 album, 3 tours and several dvd magazines (and all that work off camera) before 10ki joined. Not to mention that Mizuki had Egg experience (even if it is short). Half a year apart, still means half a year of work, and in the idol industry, that's basically work everyday. 9ki have been pushed all this time by older members to keep in mind that they ARE senior to 10ki, despite management grouping them constantly, so it definitely should mean something at least. As far as seniority go.