Nobody is irreplaceable.
The key is that Apple focuses heavily on industrial design, and is likely to continue to do so. There are lots of good designers out there. Apple has an established aesthetic, and an entire portfolio of popular products upon which to evaluate future design decisions.
And Apple has made plenty of missteps under Ive's tenure. So while we did get unibody Macbooks, we also got the G4 cube, too-soft plastics on iPod Nano's, polished backs on products that scratch all to hell, poor hinges on the first gen Air, the inability to deliver a white iPhone, etc. While you can't pin everything on Ive, you also can't ascribe all success to him either - a lot of smart people contributed a lot of excellence to stuff like the milling required to deliver unibody machines.
If Ive leaving scares you, the glaring user interface inconsistencies and poor design choices creeping into OS X and iOS unabated should absolutely terrify you. Just look at iTunes and Facetime as examples of two brutal departures from the human interface guidelines. Or the truly terrible calendar app built into the iPad (I'm still bitter about not being able to customize calendar colours, and those horrid little scroll arrows for moving fowards and backwards days)
The whole thing smells like an unsubstantiated rumor designed to make someone some money on shorting APPL.
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-Scott
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