You're forgetting input devices and UI. Go ahead, try and write a thesis on your iPad. You'll see why PCs will always be superior pretty quickly.
You're missing the overall point (which, admittedly, wasn't very well made): a tablet fulfills (or will fulfill) the needs of the majority of people. To go into more detail:
1. A "majority" is not "all". No sane person is saying that a tablet will, in the next few years, fulfill the needs of everyone. However, as most people just consume media and data (web browsing, email, IM/messaging, etc.), a tablet works just fine.
2. No, a "majority" really is not "all". Not everyone simply "consumes media and data", and a tablet won't work for them. Examples include people who do significant software development, people who analyze data (spreadsheets), and people who heavy-duty number-crunching simulations of varying types. There are many other examples. Of course, a tablet won't work for them. (I suppose I should include hardcore 3D games, too, as the keyboard/gaming keypad/mouse input combo is still a much superior UI. :-)
3. While there are many examples for which a tablet isn't good, your thesis example isn't very good. As other people have pointed out, there is such a thing as a bluetooth keyboard, and a tablet & keyboard will work just fine for a thesis that doesn't have a lot of pictures, plots, and diagrams.
4. No, the PC isn't going away, but it's long-term marketshare is going to fall into the toilet. Like the horse carriage, blacksmith, and buggy whip when the automobile came out, the long-term outlook for PCs isn't good. (And, just for emphasis, let me point out that horse carriages, blacksmiths, and buggy whips are still made today -- just not in the numbers of yesteryear.)
5. Don't ignore the potential of "AI". For example, today, Siri is like a 1-year-old baby. Wait until it grows up.