I have a feeling that Nonaka will be the next out the door...
...could also pull an Aika and stay with H!P for a while until she's fully certain of her future. I just don't see that being as likely...
I'm totally gonna give myself a half-credit for that, even if doing so is barely deserved. At the very least, this gives me some confidence that she'll probably still be in H!P in 2020 whether she stays in the group past December 2019 or not.
^I saw it the other way around. If Erina were graduating, I don't think there would be as much of a reason to try and rope Ayumi into it this early instead of waiting to use it as an announcement in the grad concert. Assigning Ayumi as co-subleader with Erina after both had previously stated reservations about this scenario, it makes me think that Mizuki may be considering a grad. If the transition of Erina to leader and Ayumi remaining as subleader happens sooner rather than later, it's not a scenario that is prolonged for much longer than either would have wanted. Having the announcement made for Ayumi's "promotion" to subleader before Mizuki announces would also destroy any wiggle-room that Ayumi may have had about not wanting to be subleader after Mizuki's graduation. The alternative in that case would be either Maa-chan or Sakura, and skipping over anyone for either age or seniority would just cause speculation about future grads that may or may not be immediately on the horizon.
If they skipped over both Ayumi and Maa and made Sakura the subleader, it would be too indicative that at least one would likely be graduating by the end of the same year. If they skipped over all 3 and the subleader position went straight to anyone from Nonaka, Kaga, Maria, or Morito (these H!P, MM, CG seniority loops, I swear), people would be expecting a massacre by the end of the year.
That said, if Erina becomes leader this year, I fully expect that she will be in the group for over a decade, and quite possibly pass Sayumi's tenure just in time to graduate a month before her 26th birthday.