"Dependent" probably is not the best word. It connotes a certain vulnerability and ineptitude when the upswing in iPhone and iPad sales as the bulk of Apple's profits was by design as a replacement for the coming iPod sales decay. If Apple was a slough of a company, you'd be right, but it's shown to be quite nimble and graceful easing out of a declining product and going to the next big thing. If Apple is dependent on anything it's not any one product, but it's brain trust: Cook-Ives-Forstall-Cue-Schiller-Mansfield.
Indeed! For the most valuable company in the world, Apple probably has quite a low 'bus factor' (i.e. the number of people it would take to be hit by a bus in order to put a project - or a company - in trouble).
Much of that seems to be because have quite a small, focused engineering group. That explains how they keep their R&D costs low, and let's face it - throwing headcount at it won't necessarily make a better iPhone. "9 women can't make a baby in a month.."