He probably won't be replaced (at least not completely by one person) but -- and this again ties back to the Apple University effort -- it will be even more impressive if the crux of what makes Jobs/Apple great is successfully imprinted on dozens or hundreds of Apple execs and engineers.
It's not just the decisions that Jobs makes that are important, but the fact that a decision coming from Jobs means something to every employee at Apple.
Ive+Forstall are within a couple of orders of magnitude from Jobs (the only people who are even in the neighborhood of reverence). But the fact that they're two people, with two egos will probably mean that the two together as the visionary aspects of the company won't be as unified as a single Jobs.
Essentially, Apple is a one product company. That product is "the mac". The "mac" comes in the form of a combination of software and hardware. OS X on Macintoshes is the past, iOS on iDevices is the future. All of that is Forstall (if I'm understanding things right.) And even then, the hardware is merely the box for the software. They make a very great box.
But Forstall is the "new jobs" in that regard. Ive can demand that it have fewer buttons, and Schiller can demand that marketing be on message, and Forstall has to listen to them, and if he doesn't then Cook should have their backs. Cook is thus well placed to be the "vision guy" (and yes, I think that is a legitimate description and that he'd be just as good as jobs at it) whose got his finger on the daily operations but also is looking 5, 10 & 15 years into the future.
Apple University is the glue that holds these four guys together and keeps them on the same page. Plus, of course, The Writings of The Chairman from Jobs over the years.
In fact, I would bet good money that there literally is a book within Apple, that probably only has a dozen or fewer copies, written by Jobs about the Apple Way and presenting his vision for the Apple Way. In a way, his living will to the future.
I think this book is Apple's equivalent to Cokes "secret formula", only Apple doesn't trumpet that they have a secret formula, they keep even the existence of it secret.
http://www.npr.org/2010/12/30/132488837/The-Behind-The-Scene...
Apple has some amazing designers and engineers in the likes of Ive, Forstall, etc. It doesn't need to promote one of them to the top spot in order to extract the genius from them.
This is why Cook wins by default. He's a CEO Wall Street and employees will accept, and nobody's expecting him to have a Jobsian design aesthetic or artistic vision. But they're expecting his lieutenants to carry that torch while he keeps everyone and everything unified.
It depends on your definition of "product", but most would classify the iPhone as a different category of product from a mac, not just a different product. As different as a minicomputer is from a desktop. I think before long we'll see Apple launch yet another new category.
Even if we forget the special talents of Steve Jobs, most companies go downhill without their founder/s. Probably HP most famously. It's partly that others lack the unique clout of founder, as kenjackson says. In addition, founders tend to be insightful, energetic and bold (else they couldn't have succeeded as founders). Employees generally don't have that crazy extreme fire, or they'd be founders themselves.
But Jobs is unique beyond that, because it's rare for even founders to create more than one dominating industry. Jobs has done it with the Apple IIe, iPod, iPhone and iPad. It's that he can see what's important; and can make it happen.
I think his design talent/taste is in addition to this, and while yet another unique talent in its own right (and related), is not quite as important.