You're sad because Miki is leader? If anything, being leader almost guarantees Miki will still be in the group for at least another year or two. Unless of course she just decides she wants to leave like Mari. Miki will eventually graduate regardless of whether she's leader or not. I don't see her graduating out of H!P unless she wants to settle down and find a husband o_o . Besides it seems she actually does love her job, and I know she loves singing so I don't think she'll be leaving H!P anytime soon.
Avex will not take Miki, they have no reason too, she also doesn't have the best reputation >_>
Now let's stop worrying about what we all know is going to happen sooner or later and just enjoy our time with Miki.