My premise is that tablets will eventually replace 80% of computer sales. As with Apple computers, one size doesn't fit all. They would have to really, really be ignorant of the needs of their customers to ignore these simple facts. And they're not. I wouldn't (and didn't) say the iPad has reached the end of its evolution, or that there will never be any other size. As I said in the post to which you were responding here, they will release different sizes of iPads if and when they have developed compelling products in those sizes.
The iPad isn't a toy or a novelty it's the (near term) future of computing. Thinking its reached the end of its evolution is crazy. It's like saying the Ford Model T is what cars will be forever.
I'm not sure computers with physical keyboards will become a thing of the past, however, and thus I'm not sure you're correct about tablets replacing 80% of computer sales. Present-day notebooks are marvelously functional in ways that tablets, so far, aren't. The clamshell design, which has persisted for a very long time, has done so, I think, because it functions well. When closed, the screen and keyboard are protected. With a flick of the wrist, the device is open and you're ready to work. Tablets don't offer those advantages.
Take Microsoft's Surface, as an example. It has that nifty keyboard cover, and a kickstand, which makes it kind of look like a notebook computer. But that kickstand allows you to set the display at exactly one angle, unlike a notebook, and the keyboard will probably have a pretty lousy feel, because there's no room to allow for a comfortable amount of key travel.
I'm not saying that we've reached the pinnacle of computer design and that all future designs will be identical to the present ones, of course. But a new design replaces an old one only when it becomes more useful.