You have to say why it will decline. You have to say whether it will be replaced, and if so by what. You have to present your reasoning and justify it.
Microsoft's then CTO presented his reasoning and justified it. So should you.
(And if you're going to whine that you were just making a joke? No. You were spewing a cheap little piece of cynicism.)
But that's a prediction heavily dependent on timing (both those concepts failed to take off) and IMO misses what actually got the next generation of computing to take off (wireless networking, high-res screens and capacitive touch) and it also misses what is the main thrust of the next generation, mobility rather than connectedness.
People are predicting that mobile devices will be replaced by implants, or one of the many alternatives currently being prototyped, for reasons trivially linked to the relative strengths of the replacement. Check out Engadget and similar outlets for such predictions, they are dime a dozen. Imagine one of those articles nailing it; if you don't look at the other crap on Engadget, it will look like they actually knew what they were talking about.
Hence, no bullshit.