I first saw the news about this yesterday morning, however this weekend has kept me occupied and I've not been able to really think or comment on it until now.
One thing in particular that I noticed, I first started getting into MM/H!P just after the 6th Gen had joined. Koharu was the first MM member/generation that I've seen completely from the beginning with the audition process, and once she graduates, she'll be the first member for whom I've seen their entire tenure in MM. While she was never my absolute fave, I still liked her and found her entertaining. She was fun to watch and you could tell (in her own spazzy own way) that she genuinely enjoyed being in the group.
As far as her graduating goes, granted we all know that it is something that's bound to happen when you get into this fandom, so on that note alone, it's not really surprising. We all knew that it would have happened sooner or later, and that the same will hold true for any/all of the other MM members. The surprising thing for me personally is the timing of it (namely, the fact that Koharu is still included in the upcoming 2010 MM Calendar leads to the belief that this decision was NOT something that was being considered until recently). Another thing was the translation of Tsunku's comments where he talks about her starting a solo career, but then needing more lessons first. This is definitely not the attitude that one would expect if you're planning on launching a soloist. If anything, one would expect UFG/UFA to have her doing the added lessons on the side, keep her working in MM as long as possible in order to "maximize her use" in the group, and then once she's deemed ready, to get her solo career going as quickly as possible so that again, they can start making money off of her as quickly as possible.
In the end, however, we'll likely never really know the real reason(s) why this happened now. For all we know, she really did want to get into modeling. For all we know, there could have been a sudden, recent, REALLY good offer for a modeling contract for her and Koharu & UF* had to make this decision really quickly. It might be something similar to the rumoured reasons for Konkon and Mako's graduations, or it might be that shi's hit the nail on the head with her theory. Bottom line, right now we don't know, and chances are we won't ever really know. As fans, we still have to deal with it. Personally, yeah I'm bummed. Like her or hate her, one couldn't deny that Koharu brought a unique flair to MM. No one in MM could freak out about eating a Chinese dessert of candied frog or squeal in pain at getting her cheeks pinched quite like her. Whatever road she ends up having to take after that last concert, I hope for the best for her.