Chinatsu hasn't had that much of a role overall in the series, in the end of the day Nakano is the only real option outside an harem ending.
being honest about things Nakano is also the one who is sexually active outside Nakano's club activites so eventual harem ending feels much more suitable for the series.
lets be honest here, if Miyako actually picks someone it will still end up as an open relationship because that girl gets sexually assaulted 3 times a day and ends up rolling with things like its nothing every time...