I'm sorry but I literally busted out laughing at your first sentence. I have two machines here in my office. The real engineering work gets done on the Sun computer that runs Red Hat Linux. The Windows laptop is mostly for checking email and running MS Office applications.
When a list of things that "most people do" on PCs consists of only leisure activities, what it says to me is that the list is not an exhaustive list of their needs. In fact, the list may not even account for the majority of their Internet activities, on any given day. Unless they are retired, and even then I would wonder if it did.
The tablet will only replace the PC when the tablet becomes a PC. Which, of course, the Surface Pro has essentially done. Develop a decent docking station, allow the use of big or multiple monitors, so you can have multiple windows open at the same time, and then you have merely created a more portable PC.
The data is about new sales and not usage. Even if the PC usage remains stable, eventually new PC sales had to stop growing. And there are plenty of people who have PCs and some who would bever buy one. Those people are buying tablets. And many only need one PC in their household, but want a tablet and/or smartphone per person.
I have a 2-year contract for my mobile phone here in the UK, because I'm happy not to have the latest, wizziest phone. Many/most people here have 1 year contracts, so get a new phone every year. Given the number of phones in the world is already over 6 billion, there's your 3 billion reached easily:
A doctor friend of mine in Chicago does this but his old phones are passed down to his sons as part of a family plan (Mom has her own plan, from her work) at the end of each AT&T contract. Dad has a 5, the youngest boy has a 3GS, the oldest boy has a 4S (and the first 3 phone is at the bottom of Lake Michigan).
The trend toward tablets (huge smartphones) will continue.
It is true a tablet cannot substitute a workstation. But most people do not need the compute power of a workstation. Just look at what most people do with their computers.
1. listening to music
2. social media
3. watching videos
4. web browsing
A tablet is good enough for these tasks and it is
1. mobile (just 1 pound, or even less. I personally do not classify a 3 pound laptop as mobile)
2. easier to use (mostly due to the apps are not (yet?) overcrowded with features)
3. easier to maintain
I find this attitude revolting, where you can throw away a couple hundred bucks every year, filling the landfill, for some shiny object the marketeers want you to buy. To equate EETimes readers to the world population would be a mistake.
If you can do research projects for school or for work, if you can write papers and process photographs, if you can create spreadsheets and write and test software, if you can manage (not just check briefly) your bank accounts and your investment portfolio, if you can do your taxes, on the machine, then in my book it qualifies as a PC.
On the other hand, if you can only do such jobs in a half-*ssed manner on the machine, if at all, or if you only really feel comfortable reading books, or the newspaper, or seeing the local weather report, then I would not classify that machine as a PC. Nor would I expect it to be able to replace the PC. Augment it, sure. But you're always going to have that PC back there, somewhere, unless someone else is doing all the real work for you.