I think I could make a list for now of both!
Asuka Fukuda - I think just for the fact she was so talented and young and left at their 5th single was enough to qualify, but as Farrah said, it is a bit difficult to imagine a timeline where she was in the line up for songs like "LOVE Machine" and further on....she was just that mature of a voice (then again Yuko, Kaori, and Ayappe too seemed to be ok)
Aya Ishiguro - I would say she left too soon just for the simple fact that she left Tanpopo more. I dunno why but I seemed to really enjoy 1st Gen Tanpopo the most and wasn't fully into the cute shift once Aibon and Rika joined. Not sure about Momusu since she was more of a supporting member to me, but I felt her graduation was a bit of a bummer
Hiromi Yanagihara - Doesn't need to be explained
Azusa Kobayashi - I mean I kind of get why she left, but I kind of played the what if scenario here I wonder if she hadn't had left maybe Country Musume would've been different.
Danielle - I think she left too soon almost on the basis that she was an amazing singer during her tenure. I'm not sure what could've happened had she not left, but I don't know
Lehua - 3 singles, but I don't feel quite as bad letting her go since she wasn't near at the level of Danielle, Mika, or Ayaka
Taiyou to Ciscomoon / T&C Bomber - Just so much talent gone after almost 2 years of activity. I really liked them and wished they had all continued longer than what they did.
Sheki-dol - I was more curious to see where this group would go because they were pretty good too...but I didn't think they'd disband so immediately after a major debut.
Chinatsu Miyoshi - 3 singles was not enough IMO, despite not being produced by Tsunku, I don't know what her future could've been within H!P be like (maybe she could've been like the same route as Michiyo or even Yuko post-Enka singles)
Maki Goto - Just in the Momusu sense...I think having her focus on her solo was a good idea, but having been in only 8 singles (and 3-ish albums) doesn't seem long enough for me. Plus she was the group's biggest deal at the time so I don't know what the company was thinking letting her graduate early on that sense.
Rika Ishii - She had potential and could've been placed in a group and not be the "Douyou Pops" girl...I felt like her tenure was just kind of a waste of time.
Miuna Saito - I feel like she got shafted in the long run being only in 3 singles and basically left to rot in Country Musume, It makes me wonder why she (or Kimura and Satoda) were just never sent to anything post "Shining Itoshiki Anata".
Ai Kago & Nozomi Tsuji - Just in Momusu, I think they could've done a couple more years in the group since they were still pretty young, but the focus on W made sense
Yuki Maeda - I may love her music, but I felt like she stayed way too long for performing such a niche category of music (which if I recall wasn't really selling all that much to begin with). Didn't help she only like released one single a year xD (except 2004). Still having to drag her career up to the Elder Club graduation was just not helping much. Great songs, but those singles could've been released in maybe 3 years time and not 8 years.
Kaori Iida - Yeah, too long, but it didn't feel intrusive to me that she was in Morning Musume for as long as she was.
I'll continue this later!