There are lots of reasonable questions about iPad's growth and future. Nevertheless, this article makes a nice argument that Apple has enough of a lead in the space and, more importantly, a sustainable advantage in processors, to continue to earn profits and improve the product without obvious near-term risks or competition in its high end space. This is a direct result of Apple's vertical integration and, as a corollary, having a proprietary source of supply for chips. Whether or not there is some enormous use case of the iPad is still somewhat unknown (to be clear, it is a big business even if people have difficulty articulating exactly why it is better than alternative formats), at least there is time for Apple, its customers, and its developers to figure it out.
Even more interestingly, the article notes that this processor independence makes a strong case for AppleTV and AppleWatch. In brief, as Apple works to bring general computing to different places, it can stake out a high and differentiated ground using hardware (processor) and software (OSX/iOS) advantages unlike anyone else. It is not the idea of vertical integration as a defensive strategy that is interesting; it is the fact that no one else seems to be trying this (at least successfully).