I see why everyone's disappointed that Yacchan seems to be slipping away. I do agree that the scenes with Toono and Yacchan are quite adorable, they'd make a good couple. I see the vision. But if you actually look closely, the way Toono acts around Kashima and Yacchan says a lot. He knows that both of them like him, but you can see the big difference on how he acts around them.
I can't really describe it well due to the fact that I literally haven't read this manga in so long- But what I can say is that the air around Kashima and Toono have more of a romantic vibe than Toono and Yacchan. I believe Yacchan was able to grow and have his like, character development thanks to Toono, but their "like" for each other don't seem to align very well. Although Yacchan does like Toono, Toono's like is far different in a sense if we compare it to his "like" for Kashima.
It seems for Yacchan and Toono, Toono feels a little compliant because he's aware of Yacchan's feelings. He's flustered and feels swept away with the moment because of the tension. But with Kashima, Toono initiates and goes with the flow of what his heart feels at the moment, he even considers his feelings for Kashima quite carefully and wants to understand it. He does also gets flustered around Kashima, but in a way that he feels shy and aware of his actions towards him.
So it's like:
Yacchan -----> <----- Toono
Kashima -----> (?) <----- Toono (SInce he's thinking carefully about his feelings)