I agree with you that it's not possible, but not for any of the reasons that you listed in your sizable blog post. Most of the things that you listed could technically be combined into a single, extremely expensive device. But the reason it is not possible to make "the perfect computing device" is not technical, it's philosophical.

General-purpose computing devices have such a wide range of uses and every day humans are coming up with so many new ways to use such devices, that it does not make any sense to cram them all into a single "perfect" device. "Perfect" is relative. Perfect for whom? A Ruby programmer who likes to hang out at Starbucks? A graphic designer who juggles $10K worth of Adobe software? A photographer who backpacks around the world, often in rugged terrain? A scientist who needs a way to visualize the latest data from his particle accelerator?

Give me the most perfect Ultrabook that you can find, and I'll still complain that it doesn't come with multiple screens. Give me the most perfect tablet/keyboard combo that you can find, and I'll still complain that it only comes with 1 terabyte of storage. Or that the keyboard doesn't have a numpad. Or that I only get 30fps on my favorite game. Or that the pixels are too large. Or too small. Or whatever. Because anything you think is good enough to count as perfect will be seen as terribly deficient by a lot of other people who have different needs, and vice versa.

For a while, I hoped that the success of data sync tools such as Dropbox would make ideas like "the perfect computing device" irrelevant. Because people in wealthy countries would just buy several devices, from 27" desktops to 7" tablets to 11" ultrabooks to 3.5" iPhones, sync them all, and use whichever device suited their needs at the moment. It would be the combination of all of them and the way a person makes use of them that would be perfect, not any individual device. But it seems that some people are still looking for the One Device to Replace Them All.