Meh. Unfounded fears. There will always be computers/desktops/laptops. Just because tablets and smartphones are ubiquitous doesn't mean that they won't exist.
People will always want huge monitors, and there will always be programmers. Granted, in 20 years it might not be someone sitting in front of 3 LCD screens, but it will be something.
Sure, but desktops and laptops are starting to become more and more like tablets and smartphones. Look at the changes that started in Lion, especially the Sandboxes. Look at what is in store for us with Windows 8. It seems to me that if things keep heading in the same direction, laptops will end up just as locked-down as tablets.
If you think that the market will never let it happen, realize that at least 80% of consumers only really use their computers for the web and/or for games. What do you use your smartphone for?
People will always want huge monitors, and there will always be programmers. Granted, in 20 years it might not be someone sitting in front of 3 LCD screens, but it will be something.