Unless, of course, the people he's hired follow the same hiring practices, and train them in the same way. As the saying goes, A players hire A players (and in this case, one hopes A* players hire A players, then make them into A* s). I don't buy that companies are built around one person, and then revert to the mean - it's more about setting up a culture and following it. Does Google do well because Sergey is wandering the halls and jetting around the world shouting out inspiration? Or is it the culture that was created, covering it's many offices, that delivers the goods? If that is allowed to die, then yes - the company stagnates. But there's no reason to believe that the executive team can't keep it alive if they're as good as they should be.
That's not to say that Steve's absence isn't a big deal - it's just not a "Dump your stock, it's all over" level deal.
Leadership isn't just a procses...it's a vision. It may very well be that somewhere inside of Apple there is someone equal to Steve Jobs in terms of vision, determination, and smarts. That's not likely, however.
Companies like Apple fall off of their perch because you can't turn greatness into a process. It's much more innate than that. The grandparent is right. They'll be ok... for awhile. Soon they'll go the way of HP and IBM (and increasingly Microsoft). Not gone... just not what they once were.
That's not to say that Steve's absence isn't a big deal - it's just not a "Dump your stock, it's all over" level deal.