You know, if this really is the angle the mangaka was going for, they did a pretty poor job of communicating it. Or it's somehow lost in translation. By that I mean Hinaki really only seeing him as a best friend and not developing feelings. If it really is the angle, I think it's original and very interesting. The sheer fact he straight up told her of this dilemma is already something I don't think I've seen before in this sort of work - someone telling their best friend they're scared they'll drift apart if he gets a partner.
I think it's maybe conditioning from manga in general and how if you got a male and a female character who are close, they nearly always develop feelings. I think it's partially because boys and girls don't tend to become besties in HS, since it's usually gendered circles. Readers are just so used to this nonsense that people will naturally fall for each other rather than staying friends that it becomes difficult for a mangaka to make it clear this is not happening while not being too upfront to the point it ruins suspension of disbelief.
This manga continues to elude me in how it might be really good but at the same time isn't. Like the author is walking a tight rope between everything being really well characterized and the exact opposite with characters not being clear to the audience.
Thought from everything so far, I now personally lean on the later and just think the author has big issues communicating their character intentions to the reader. Makes it too easy to insert our own best case scenario and bet set up for disappointment.