Members like Yossi (and in the past, Nacchi, Kaori, etc) don't "fit" into Momusu anymore because the generations are getting younger and younger, minus Miki of course. It's almost a bit strange to see adults being surrounded by 'kids', but at the same time, an adult is exactly what's needed to lead the group.
I'm still waiting on that official announement, but if it is true, then I wish I could send a huge thanks to her for leading this group so well and keeping them focused; even with all the pressure put on her, with Gatas members leaving too and whatnot.