The rise of the smart phone and tablet happened in the same decade as when desktops and laptops reached their saturation point. Where most people are no longer buying a desktop for the first time, but replacing their old desktops. Thus slowing sales.

The Smart Phone isn't killing the desktop, and more then the desktop killed the main frame, we still use powerful dedicated computer clusters for computing, just like we did in the 60's and 70's before the desktop. The desktop just expanded the market of people who own computers, much like the smart phone is doing now.

That being said I do most my reading on a desktop.