This is a recent trend I have been noticing recently in Kamen Rider, a villain dies or soon gets killed when the secondary rider is shown in the series or is coming up. I first noticed this when Kirihiko died in Kamen Rider Double and the next episode was with Accel. I liked that though because one character is gone so the show tries to fill in the gap. This continues though with OOO as Birth's first fight in the series was when Mezool and Gamel fused together with many cell medals. In that fight he just blasted them and OOO finished the two off with Gatakiriba. They were brought back into the series, but killed later on. Now even Fourze does this because of Sonoda's death, but as I said before I'm not a hundred percent sure, but she is off of the character chart on the website. This could mean she could come back, but I don't think anyone can escape from space. I even bet this trend continues on in the past with any type of toku. I will update this post when I find out and you can help me with this as well if you want to.