Or for that matter how about leaving only Oreki and Satoshi would have been enough? You know the detective and his sidekick to make it seem like the detective is not talking to himself. What does Chitanda provide to the plot beyond baiting Oreki into solving mysteries by being all moe moe over him? If Chitanda ceases to be a romantic interest I don't see any utility in her being there. I mean sure it's her uncle's notebook but it could have been someone else's uncle.