The oshimen thing to me was just another way of saying my favorite member. I see myself as a casual fan, so I don't get into that deep stuff.
I am a fan of AKB48. My oshimen is Oshima Yuko. If she graduates, it would suck, but in the end, I don't care lol. AKB48 is so massive, so many personalities that it's easy to find someone I'll like, so of course I'll still be a fan. Though when I was into H!P, I slowly ceased being a fan after Ogawa left. However it was also because I felt there was a decline in quality and it just didn't feel the same after that. They weren't everywhere like they used to be anymore, and it was harder to find content(like TV stuff). I was a lazy casual fan, I guess. AKB48 is all over the place now, it's hard to avoid them, so easier for me to keep up with stuff, lol.