pulled this from an old Time article on Cook
If we examine what's happened to Apple's stock whenever Steve Jobs has taken a leave of absence, a pattern begins to emerge.
In 2004: Cook filled in for Jobs in August and September. On August 6, 2004, Apple's stock was trading at $14.89 per share—its lowest level in months. By October 8, 2004, the stock was trading at $19.53 per share. By the end of the year, with Jobs back, the stock was at $32.20 per share.
In 2009: Cook filled in for Jobs from mid-January until the end of June. On January 16, 2009, Apple's stock was again at its lowest level in months: $82.33 per share. By July 2, 2009, the stock was trading at $140.02 per share. Apple finished out the year at $210.73 per share.
So Steve Jobs appears to have a positive affect on Apple stock while he's in charge (or when he comes back from a leave of absence) and a negative affect on the stock when he initially goes on a leave of absence.
However, both times that Cook has filled in for Jobs, Apple's stock has gone up. Perhaps not as astronomically as when Jobs is in the driver's seat, but up nonetheless. It could be argued that if Jobs were to leave Apple altogether, the company may not grow as outrageously as it has in the past. But it's very unlikely that the company would crash and burn.
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xyzzy