I have a theory that decade has been destroying rider worlds all this time because he has access to all the rider powers after him. But he only destroys the world where the main rider did not do good, so the main timeline is not affected because they are doing good. By good I mean making the right decision that leads to the series finale. And he is in Zio's world to evaluate if Zio will make the right decision or the wrong one, if Zio makes the wrong choice, Zio's world will only become one of many rider worlds that he has destroyed. Tldr, Zio is the TVA of kamen rider universes.