Have mainframes disappeared?
Have minicomputers disappeared?
Apart from being renamed "servers" and "clusters" and such, they haven't. They don't rule the roost to the extent that they used to, but the desktop didn't kill them: they found their niche in doing things that desktops weren't suited to.
So it will be with desktops compared to laptops: they'll handle the computing tasks that aren't practical with laptops (gaming, media editing, etc.).
And so it will be with laptops and netbooks and such: they'll handle the tasks that tablets and smartphones aren't suited to (e.g. writing code for the tablets and smartphones)