I get why people aren't the happiest with this development, but I kinda like it. It's an interesting development from started appearing as a one-note revenge story. Rather than revenge, it's a story about how people's insecurities can come out horribly and hurt those people you care about.
For the ex-girlfriend, she's insecure about how much he likes her, because the guy's kindness appears to be putting others above her. In turn she tries to stoke his jealousy to get him to declare his love for her and to fight for her, proof that he truly cares for her above others. It backfires.
For the kohai, she's insecure about the things she likes and preemptively disparages them so she won't be brought down herself. She found solace with the protag and was able to speak to how she really felt. But as she hears about his heartbreak and the fear of being rejected for what she likes, him, she falls on her old coping mechanism and talks down on what she likes, him. It backfires.
And while I'm not sure about his Oshi, I feel like it's going to be about how she had no self confidence and didn't know how to express herself, and he helped her open up with his kindness. Maybe helping her with the dom, bullying type character that speaks her mind. But she still the shy person inside, so the only way she can express herself is through said bully personality by giving him a "higher" pedestal of a slave, rather than everybody else being a pig. She tries to give him special treatment by showering him with extra attention and "loving" insults, the only way she can express herself. It backfires.
This could very easily become a revenge fantasy book. He was nice. They were mean. He's mean back. Could be very satisfying for people who feel like they're nice and that's all. But I feel like the story already recognizes that not everything is coded with malice and not everything needs to be a done deal. People are allowed to make mistakes, and have a chance to make up for them. He doesn't need to forgive them right away, but if they sincerely show regret, maybe don't let them all the way in yet but be clear about how you feel and give them a chance.
All this to say, I like this manga and appreciate, at least slightly nuanced characters. Overly long comment over lmao.