I don't think the question here is about his innocence or guilt - it's whether he is fit to stand trial. If his dementia is advanced, it seems like there's no point. But his victims probably feel otherwise.
In general I have mixed feelings about these trials that happen decades after a crime, especially if there is no physical evidence that a crime took place. I realize there must be a pretty solid case to convict a person like Bill Cosby who is wealthy and has a great defence. But on the other hand many celebrities are ruined simply by an accusation whether they are convicted or not. Kevin Spacey, for instance.
And we have a guy in Canada, Jian Ghomeshi, who was found not guilty yet his career is over. And he wasn't accused of rape or murder. It does bother me that people act like someone is guilty because they are accused.