On one hand as the single most well-known member of the group she could keep delaying the inevitable for a little while longer and continue promoting and bringing more attention to the group. On the other hand, as much as she always tried her best to bring attention to the other members, the longer she delays it the greater the risk of her overshadowing her kouhai and smothering their growth because the public at large will unavoidably focus on her.
There's also the factor of her physical and mental limits to consider... and I don't mean this to paint her in a negative light whatsoever, it's just... she more than anyone else deserves to have some relief and peace of mind after all her contributions if/when she felt she needed it
<for completeness sake, I'll acknowledge it's also likely that her having future plans that no longer involve H!P/MM could also be huge factor. But since we know next to nothing about it I'll consider it when we have more concrete statements from her/her management. On the contrary though, I often read and listened to her emphatically saying Morning Musume is her dream job and considering her somewhat laidback personality career direction-wise outside MM... I personally doubt it factors much into her consideration. She even said so during one of her mc at her graduation announcement, that she haven't decided on what she would do after this. If there's any justice in the universe maybe Tsunku/the management will convince her (or her convince them) to still do MM-related things like maybe assistant producer? guiding the 12ki as manners coach, public speaking trainer, stage presence improvement adviser? lol I can dream>
And I think here she succeeded rather brilliantly as well
She made sure her kouhai has matured enough and ready to stand on their own before graduating at a high point for the group so that the public would still have reason pay attention to them even after she's gone.
As a long-time fan it will always be too soon for me... I had hoped her leadership would extend longer than this, that she would have her day in the sun for as long as she spent in the shadows at the beginning of her tenure more than a decade ago. That she would be at the forefront of the group she loved so much and given so much for, leading her kouhais for at least 4 years, to match the longest period of the Morning Musume leader's tenure before her. Alas, it shall never be.
But I trust her judgement in this.
After all the hardships she persevered through in the group's darkest days and the many times the pressure and stress got to her health, considering all she managed to accomplish despite of that... Sayu would know when the time is right the best.
p.s there's a 12ki audition thread if you guys wanna talk about that you know.....
