However, I'd disagree that the theatre is "for unpopular girls and birthdays", it's still the most important thing. You could say, when a member stops appearing in the theatre it's time to graduate, which happened to Mariko and Yuko... Kojiharu is the next one who basically never appears...
The theater is more like a cocoon. When a member blossomed into a hatsukoi butterfly, she spreads her wings to fly towards her dreams.
I.e. appearing on prime time TV shows, solo singing career, movie or dorama shoots, modeling, CM deals and whatnot.
The (unpopular) caterpillar idols never reach this stage and are stuck in their cocoon.
Mariko and Yuko spread their wings and flew away for good, but girls like Mayu and Jurina managed to achieve hatsukoi butterfly status much earlier in their lives, allowing them to leave the cocoon behind without the implied motive of graduation.