While I'd overall agree with the "too much flashback!" sentiment I think people are a little bit harsh over the current arc. Sure, if it's only three chapter to the end from here it'd suck, but if it's planned to be a long series it's nothing that outrageous and clearly these events are important for Asuka's ... teenage formation years ^^
I feel the same way, I think it gives the characters more depth and shows how much thought the author has given to this series.
If Senpai is plan to stay and still play major role that's said because there no point to give that much backstory if you gonna ditch the character after or giving her minor role.
I'm probably reading too much into this but the way I see it, Senpai serves a few purpose to build up Asuka as a character. It feels like she is there to show what a 'wrong' kind of 'love' looks like, given how Asuka has probably wrestled with how 'wrong' her feelings are for her mum.
It seems as if what happened with Senpai was a catalyst for Asuka to truly accept her feelings and be certain and not as horrified that she is indeed in love with her mother. So even if Senpai does only appear for a few chapters, I feel like her role is important for Asuka as a character.
But to each their own I guess, I do understand wanting more Asuka and Ayako moments though.