How many of these "tablets r great" opinion pieces is Doug going to write this week? Netbook sales declined slightly, that does NOT indicate that they're going to disappear completely. Everything has it's market, and netbooks are still far superior to tablets in actual productivity.
Desktops and laptops have lost a lot of ground compared to where they were, however their extra size allows for a lot more hardware, so they will still have a place with gamers and other dedicated people who need the processing power or screen size. Smartphones and tablets, which are just smartphones with larger screens, are good for portable media and web browsing, but the lack of a physical keyboard prevents them from getting serious typing tasks accomplished. Netbooks have lost the sales of browsers and media fans, but they were never really made to handle HD media. You also can't argue with the price, considering netbooks still can hover around 300, whereas tablets hover more around 600.
The market has diversified, but everything still has it's place. The near future has a lot of tablet/netbook hybrids, which blur the lines between these markets further, but the only thing that's really lost it's market space is portable media players. Smartphones and tablets have indeed made such devices obsolete for any reason other than price.