Jony Ive was "promoted" to Chief Design Officer a while ago. I say "promoted" with quotes because many people believe he was moved to a position where he can do less damage. Jony is no longer in control of Industrial Design or User Interface Design at Apple. Those disciplines now have new heads: Richard Howarth and Alan Dye and they don't report to Ive. So to answer to your question, Jony leaving Apple will have zero impact.
I think there is a public image component to Ive as well. The image of Jobs' era at Apple is Ive and him running the show. As great as Tim Cook is doing in the CEO position, Ive is still seen by many as the remaining influence of Jobs. Regardless of his day to day responsibilities having him at Apple in some design role allows them to keep that perception. If he were to leave I think it will have a very negative impact on stock.
Good question, Caprinicus. The answer is, the kind of damage Jony has done since Steve Jobs' passing.
Let's consider Jony's performance on software design first. This is what some prominent people have said about iOS 7: The Verge wrote in their review: "iOS 7 isn't harder to use, just less obvious. That's a momentous change: iOS used to be so obvious." In iOS 7 basic usability features such as making buttons look like buttons are now stuffed under Accessibility options. About this, Tumblr co-founder Marco Arment wrote: "If iOS 8 can’t remove any of these options, it's a design failure." (And iOS 8 hasn't.) Michael Heilemann, Interface Director at Squarespace wrote, "when I look at [iOS 7 beta] I see anti-patterns and basic mistakes that should have been caught on the whiteboard before anyone even began thinking about coding it." And famed blogger John Gruber said this about iOS 7: "my guess is that [Steve Jobs] would not have supported this direction."
And what about Jony's other responsibility in the last few years, industrial design? The iPod, iPhone, iPad, MacBook Air and other Apple products are all amazingly well designed and breathtakingly beautiful. But these products weren't designed by Jony Ive all by himself. He designed them under Steve Jobs's guidance and direction. Steve was the tastemaker. Apple's post-Steve products are nowhere near as well-designed.
Consider iPhone 5c, for example. The colors are horrid, and when you add those Crocs-like cases it looks more like a Fisher-Price toy than like a device an executive would want to be seen holding. Then they released some ads for the 5c, and I kid you not, one of the ads had sounds of bleating farm animals. (It was titled "Every color has a story", published on tumblr.) That the 5c didn't do well in the market shouldn't surprise anyone.
Not everything Apple does is for executives. The iPod Mini came in crazy colors too and Apple sold woolen "socks" for people to carry them in. This was under Jobs. They also did animal-print plastic for the early iMacs and iBooks--the products that cemented the Ive/Jobs relationship and defined the new Apple.
Steve Jobs once held a mock funeral for OS 9 with organ music and a coffin. Apple produced plenty of crazy / weird stuff under his leadership. This is one reason that speculating about "this wouldn't have happened under Steve" is a fool's errand.
