"Steve's personality is such that he had not brought up other people who could do what he does. He's the kind of person who pushes away people who are like himself," said Ted Kaehler, who worked on the original team that developed the first graphical user interface at the research center known as Xerox Parc, and he later worked at Apple in the '80s.
But some Apple watchers are reluctant to use the past as a guide. Andrew Hertzfeld, who helped develop the original Macintosh and now works at Google, says that Apple has had 12 more years under Mr. Jobs’s leadership to soak up his unique values.
He also notes that products already in the pipeline -- which analysts say may include new iMacs and smaller iPhones -- already bear Mr. Jobs's imprint and can sustain Apple for years to come. "It will take half a decade for the absence of Steve to really show up in the products," Mr. Hertzfeld said.
Some think Mr. Jobs's imprint on Apple could last even longer, perhaps for decades, even if for some reason he is unable to come back in June. James W. Breyer, an influential Silicon Valley venture capitalist, sits on the board of Wal-Mart and says the values of its founder, Sam Walton, still drive the retailer 17 years after his death.
"I can't recall a board meeting where Sam's spirit and contribution have not been cited in some way," Mr. Breyer said. "In the same way, I expect Steve Jobs, through his genius, will always represent the DNA of Apple."
After the initial shock of Mr. Jobs's letter sent Apple’s stock sharply lower in after-hours trading on Wednesday, investors were a little calmer on Thursday. The shares fell 2.3 percent to $83.38. Still, there are those who worry that Mr. Jobs's absence will have an impact even beyond Apple.
"The whole world is concerned about Apple. I'm concerned about Silicon Valley," said Mr. Perlman, the entrepreneur. "I need Apple to be harrying Microsoft. We need someone stirring the pot. God forbid that there is no one stirring the pot anymore. We'll become Detroit."