@richman
Not "angry," but more disappointed that the author threw away a really good opportunity for character development.
Normally love triangles are forced in and don't contribute much to the the overall plot. However, given the circumstances in Takamine, in which MC is used as more of an object that Takamine is possessive of and that, to his knowledge, she seems constantly abusive, having him get into a relationship with another girl, especially someone who seems as caring as Eri, would lead to some long-term reforms of Takamine's tsundere demeanor.
I can imagine some scenes where she confronts MC or accuses him of consorting with other women, and the tension between the two continues to escalate because he still thinks she's just using him because he knows her secret, and because she hasn't shown him any actual care or compassion. The series of events would probably lead to Takamine being more introspective about her behavior and how she thinks of MC as opposed to what she did, her coming to realize things like the false rape allegation, blackmail, and continued exercise of control over him is what drove him apart, and, realizing her mistake, she would rewind time to undo MC's relationship with Eri and fix her mistakes, starting her relationship with him and reconciling.
The reason I find that more preferable to the current state of the series is that it uses every character to further develop each other, and doesn't leave important plot points in the series to the side, nor does it ignore the behavior Takamine has exhibited and rather shows the consequences of it. Eri was the perfect character to do this set up with because she had a previous connection with MC and had the right demeanor to foil Takamine's, leading to a more reflective view of the situation. However, what we got essentially wrote her out of the work, wasted our time, and didn't progress any of our characters or develop them in a significant way, as well as ended up making any potential use of a third party to sway MC away from Takamine seem repetitive.
In other words, I'm upset at wasted opportunity for the series to use all the elements its setup and the tools in the toolbox to finish each character's arc in a satisfying way.