I think Jobs' first departure showed that Apple is NOT capable of surviving without him. It would be nice to think that the company has grown in that time, and changed its ways, but its hasn't. A simple look at their product line and business practices shows its still the same exact Apple it always has been. The only difference is that they now make the iPod. The only reason Apple's computers are still on the market is because Jobs' brought his NeXTSTEP OS with him when he rejoined the company (OSX IS NeXTSTEP).
Regardless of any actual impact Jobs has on the inner workings of Apple, the public face of the company cannot survive with him. If Jobs suddenly departed Apple, the drop in stock price would cripple the company to a point they could never recover from. Apple would quickly realize that without a charasmatic leader, people are going to be less willing to pay the Apple cool-tax. I don't think Bill Gates or Steve Blamer have ever convinced someone to install Windows just by wearing a turtle-neck sweater and holding up a retail box.