There is mounting evidence that tablets are, in fact, displacing PCs (especially notebooks). But "why" such displacement is occurring is what is really important.
One might argue that user behavior, in either consumer or work roles, has changed. PCs originally were "work" devices. Though most do not remember, it was a single application, the spreadsheet (VisiCalc), that created the initial demand for PCs. Over time, other work applications, such as word processing, moved to PCs as well.
Since the advent of the Internet, especially the World Wide Web, user behavior has morphed. A great many pursuits no longer "specifically" require a PC.
These days, about 75 percent of everything that users do on PCs is content consumption. Most people, most of the time, even at work, are consuming created content, not “making it.” They are browsing the web, reading documents or manipulating data. People are watching video, listening to audio and using social networks.
And most content creation will involve reading and replying to email, much of the time. That can be done other ways.
The point is that, since the advent of the Internet, especially the World Wide Web, user behavior has morphed. A great many pursuits no longer "specifically" require a PC. And less total time is spent on “work” activities.
You might say that “casual computing” has become the dominant way most PCs are used.
Scanning news feeds, browsing the web, emailing, reading an eBook, connecting on Facebook and tweeting can be done on many devices, not just PCs.
One might argue that casual computing and content consumption are, for most workers, most of the time, all they need to do.
An anecdote might illustrate how much matters are changing. People do not have access to Microsoft Office on their Apple iPhones, but they still manage to do “work.” People use tablets at work, but iPads do not feature Microsoft Office. They still manage to use tablets for work purposes, or at least people often claim they do so.
Android users and some Chromebook users do not use Microsoft Office. They still seem to get work done. The point is that people now use a range of devices to support “getting work done.” In many cases, they seem to do so, without use of fundamental “work” tools such as PCs, notebooks or Microsoft Office.
Over time, more time will be spent on smart phones, tablets and other devices, based simply on the proliferation of such devices in consumer and work markets. And it also appears more "work" will be done on tablets, smart phones and other devices because much "work" does not require all the capabilities of a PC or standard office productivity applications.