I'm not at all surprised about her leaving. She was always one of those who seemed very well studied and after having spent time with Momochi, I'm sure she realized even more that being an idol isn't everything one should do. These days especially, with young kids in Japan being so focused on needing/wanting a stable job and all, Yanagawa definitely seems like one of those who would go down that route. Much like Ogata, tbh. Idoling is fun but it isn't the future for them and unless they feel that showbiz is what they wanna do for the rest of their life, it only makes sense they're considering "real life" options. Especially if they're doing good in school.
I hope she had a fun while she was in.
It'll be interesting to see what happens with Country Girls now.