I voted no. Her leadership skills are too valuable, and I don't think Miki has the personality for it (just my opinion).
She assumed leadership under very difficult circumstances after Mari's sudden departure, and really took the reigns. I can't think of anyone else capable of doing the job so well. After all, in old days, Tsunku had much less on his plate; now there's Berryz and Cute and all the other groups and artists -- how much time does he have to devote to MM? The leadership role isn't just a figurehead anymore like it used to be.
I like to view Yossi as the wing nut that holds MM's world together. Losing her would leave a real vacume.