It's true. Considering many girls in other groups have looked into other fields Morning Musume really looks like their girls are more than comfortable where they are now. They all chose the group instead of studying (except for Masaki). Honestly, I wouldn't be surprised if the majority just graduated to marry someone.
On Mizuki: I think management is actually not against keeping her in the group. On the one hand Dawa just left, so looking for yet another new H!P leader might be annoying. That's why I think they're okay with her staying until 25. The bigger factor however is that she's actually a decent seller considering her photobooks and magazine shoots.
/EDIT: As of this day Asakura Kiki has been on hiatus for 5 months (since May 8th)