It's as simple as that. No doubt, there are many talented people at Apple, but SJ has the vision and drive, and he is the eye of the needle, that thingies have to pass, before we see them. That's what makes him and thereby Apple so special.
Jobs is synonymous with Apple. The stock will go down due to his reduced involvement, but if the products stay good and the company keeps making money, the stock will stabilize and may go up. But I'm not saying anything that everyone does not know all ready.