I think saving the investors from themselves should be just as big a concern. He is predicted with survival past 10 years, which IIRC the average age of death with someone having that prognosis is in fact only a few years shorter than the average age of death. If Jobs getting cancer means he kicks it at 78 instead of 82, I don't see why it should be disclosed immediately or at all.

Here is the text of the Commencement address by Steve Jobs in 2005 at Stanford where he announces that he had cancer.

Edit: not agreeing or disagreeing with anyone's opinions on CNN mentioning his pancreatic cancer, I'm just linking this because I was reminded of it after reading the comment about his cancer and I recall it being an interesting read.