Excellent story, very disappointing ending.
It wouldn't have been nearly as disappointing if the entire story wasn't polybait but unless it was axed it 110% was just trying to pull in poly readers and then throw it away at the last second, utterly ruining the character development and the story's coherency.
Even if it was axed they could've just had CH15 be them all get together, it would've felt a bit rushed but it'd've made more sense than this ending, or they could've had CH15 be after a small timeskip hinting that things progressed naturally to the point that the three of them start a relationship together.