As I'm sure has already been rehashed many times on this issue, there is little reason to replace the EF mount interface. EF was introduced when Canon began producing auto focus cameras and lenses, and they went with a new design over an FD extension because it was easier and more reliable to simply use a whole new mount design.
So, the question is, what new feature would be added to lenses that could not be covered in an EF extension? They're not going to change it just to force people to buy new lenses, because then that creates a complete toss up on their customer base. If I have to buy all new lenses if I buy this new camera, well then I no longer have any reason to stick with Canon, and that shiny new Nikon, Sony, or New Techno-upstart company's offering looks damned tempting...
The obvious answer as to "What new feature" is fairly simple: Medium format. As prices fall on full frame cameras and the general cost of production for a given sensor size is reduced, then chances are good that Canon will look to bring a new line of larger than full frame sensors to market, which would mean a whole new mounting design. (In which case hopefully we can see a full frame 'flex' body, which is easily configured to accept either EF or 'MEF' lenses.)
Edit: To make it clear, I see it as exceptionally unlikely for EF to actually disappear any time in the next several decades. About the only way that EF dies is if the market pushes the low end consumer to small sensor mirrorless systems, and the pro-sumer/professional to medium format. This would mean no new EF full frame bodies produces, and no more need for new EF lenses.
Canon EOS 7D | EF 28 f/1.8 | EF 85 f/1.8 | EF 70-200 f/4L | EF-S 17-55 | Sigma 150-500