Steve Jobs loved to talk about design. Jony Ive loved to watch Steve Jobs talk about design.
As we can see from his one and only appearance on stage at an Apple keynote, Ive nailed it. He didn't appear nervous, didn't stumble over any words, and had no trouble clicking the button on the remote to advance the slides.
But why bother going on stage at an Apple keynote to discuss design, when instead you can sit in the audience and watch the master do it?
Jobs was great at talking about design. For every brand new product that he unveiled on stage, the design of the product was usually bullet #1, the first tentpole feature he would talk about at its unveiling. It wasn't just the beauty of the object (which he certainly did point out), but how it was made and why it was made in a particular way.
You can see that when he unveils the iMac G4 ("Let each element be true to itself"), the iPhone 4 ("What are these lines in this beautiful stainless steel band? It turns out this is part of some brilliant engineering which actually uses the stainless steel band as part of the antenna system."), and countless other products that he and Ive collaborated on.
When it came time to put together the plan for this keynote, it would have been natural for Jobs to handle the discussion of the move away from notebooks assembled from many parts to a single unibody enclosure.
It likely wasn't Ive's idea to present during the keynote. It had to have been Jobs who pushed him to do it. And it's telling that the other person invited on stage to speak is Tim Cook, in only his second major appearance in an Apple keynote.
Jobs was about to go on another medical leave, getting a liver transplant. I bet in his mind, it wasn't so much auditioning successors, but at the very least, getting some feet wet to prepare for the future.