This article isn't about screen size. It's about creators vs consumers. I would argue that the new platforms make it easy to create software content. You only have to look at huge range of apps and free development tool kits.
The ability to create new hardware/firmware is another matter.
Are you sure the apps for - lets say an iPhone - are developed on the iPhone. I contend that they are developed on desktops and that is why things such as simulators and emulators are provided for the target devices.
There remains plenty of room for everyone to have their choice of device. Tablets will fit into one market, laptops another, PC's will be a very big part of the overall market for sometime to come, and workstations will always be needed by the super users.
Trying to predict a winner is like trying to predict the weather. In the end it is what it is!
Just my opinion.
The problem is the computer makers idea of what we were going to use computer for and how we were going to use them was always wrong. Remember the first home computers like the Timex Sinclair? The killer app was supposed to be storing your recipes on it. Like you said the original IBM PC was the anything machine. It was users and third parties who figured out all the neat things we can do with them. The more the manufacturers restrict how we can use computers the less useful they will become. The one upside, if what you say happens and the computer market is divided into consumers and creators, is we'll get the computer back. Computers used to be a cool toy for an elite group. Maybe it will be that way again.
I personally feel that the PC decline would have happened anyway and is not related to the emergence of tablets.Office 2003 is still in wide use as is ffice 2007 as the capabilities of the PC generally exceed what people really need and threfore slows down the upgrade appetite.
Most people I know have a tablet, and PC. Not to mention a smartphone.
Aside from UI design and the computational capabilities of the device, another factor to consider in choosing tablet or laptop/PC is the ergonomics. Using tablets ties up at least one hand to hold the tablet. You can place the tablet on a table and use a stand to hold the device at an angle suitable for reading, but a laptop form factor does a much better job at using available table top space. If you are away from a table top, the tablet form factor is a better choice.