Or maybe, over time, MC learns from Riley how to lean into Mia's various kinks well enough for him to achieve Mia's fabled ahegao face.I see no reason for them to stay together.
It seems like Mia has some kind of addiction and MC cannot satisfy her as a man.
The only chance for him to stay with her and get a "happy-end" - is to turn into a voluntary cuckold, and officially allow her to fuck on the side.
It seems that everything is heading for this.