Whenever Steve Jobs stops working for Apple, for whatever reason, there will certainly be a drop in Apple's stock. And then after a couple of quarters people will see that the company is performing just as greatly as it has been before Jobs' leave, and things will get back to normal. If you don't feel like weathering such turbulence sell your Apple stock right now and enjoy the ROI.
I do think that there probably isn't anther Steve Jobs out there, but I disagree that there aren't people, who can be great next CEO of Apple. In fact I think there are plenty such people at Apple.