More than 95% of the population actually have a life that goes beyond watching youtube and reading reduced versions of web sites. For example they'll have to work.
They'll want, sooner or later, to use office, to embed a movie clip into a powerpoint slide, to collect the pictures from their digital camera, to open large zipped files, to send a properly formatted email, to read a DVD with video clips from their 2004 holidays, to play a game beyond the limited subset allowed by touch controls, to make a video chat while they're working on a document, to access that web site that requires Flash or even Java.
And even when the stuff they want to do can be done on a tablet, the user experience there is clumsy and the applications are almost always limited - many essential features are outright missing because the "app" designers give for granted that the user will use a desktop PC in order to access them.