from the point of view of apple, the iphone 4 is still a best selling (though people are waiting to see the next phones coming out, but still), so why take so much efforts to redesign a new form factor?.
this is why so many companies r failing at some point bc they think they can stay forever on top of their high horse and stop putting effort in anything bc well "its selling anyway so why bother"
Steve would leave Apple in such time, if there is no big surprise going on. Steve hurt shares, fans and everybody else by leaving Apple. (I know that he has medical problems, but thats not the point here) And I think that he carefully choose this time because the new CEO will get a chance to introduce a whole new phone (that would be the mysterious iPhone 5), new China market model (8GB) and 4S as the cheaper one. This is great to show him in a best possible way...