Well, some people have certain characteristics and that is why they are in the position of the company they are in.  Steve's characteristics are what an innovator and exciter can do.  That can partly be credited since he is one of the originals in the creation of Apple.  Cook's characteristics may be an excellent business man but he might have been told beforehand and/or it's not in his nature to be "outgoing" as Steve.  So, could we then surmise the question Cook replacing Steve as top exec when Steve does retire, perhaps as interim but (with my limited understanding) I don't think so in the long haul.

Look at Microsoft as who took over the helm for Bill.  Steve's personality, character, and drive was a good fit for MS.