Well, I think that a large part of Apple has been Steve Jobs, but I think the company has grown to be such that they don't "need" him. I think Tim Cook is awesome and Steve is still on the board and will have quite a bit of say over what happens. I also think that Steve has created a machine that will be able to innovate without him at the helm. And frankly, he's been in and out and not driving every aspect like he used to that they will pretty much function awesome without him. I was actually talking to some Apple employees today on why the culture is so awesome, and they don't really seem worried about making strides without him at the helm.
Now it sucks a ton that he's had cancer, and we don't even really know if that is why he's leaving, and I'll point to this from USC's website on the Whipple Procedure. "The overall survival after the whipple operation for pancreatic adenocarcinoma is about 20% at five years after surgery. Patients without spread of cancer into their lymph nodes may have up to a 40% survival. The actuarial survival is less than 5% at five years for patients patients with pancreatic adenocarcinoma who are treated with chemotherapy alone." He had the procedure in 2004 and frankly has surpassed post people's expectations on survival. Though he doesn't look great, I'm daily confident that he will be around for years to come and will still be influential in Apple's products in the future.