You make a good point. Thus far, we’ve had two opportunities for a son to kill their father and in only one of them has the opportunity been taken. This is a significant moment in that it more fully characterises the power of the dark side and, as you mention, is a milestone with regards to the Rylo Ken story.
Speaking of which, I can’t help but feel that a movie could’ve been dedicated entirely to the Leia, Han, and Ken story to come before The Force Awakens. To much has happened between episodes 6 and 7, as was evidenced by the scene between Leia and Han entirely composed of exposition, detailing how they had lost their son to the dark side. Then again, I understand that the ages of Carrie and Harrison needed to be taken into account (it would be strange to have episode 7 depict Leia and Han having and subsequently losing their child given their years and accompanying gray hairs).