Okay, suppose that Shatner had left the series after Season One or Two. Could they have gotten by with promoting Spock to captain and bringing on a new Science Officer for Nimoy to play against?
I'm not sure how it would have worked. IMO, fans were used to seeing Shatner as Kirk, and the chemistry of the show would have suffered greatly with his departure. Issac Asimov is the person who pointed out to GR that the interactions between McCoy (emotion), Spock (logic), and Kirk (decision) is what made the characters so interesting.

Captain Spock?

