"The iPad is just a big iPod touch"
That’s the line that went around a few years ago when the first iPad was announced. As time has gone by and I’ve given the matter more thought, I realize those people were right.
As phones get larger and larger ( 3.5" iPhone -> 5.7" Galaxy Note 3) and tablets get smaller and smaller (10" iPad -> 7" Nexus 7), it becomes more evident to me that they’re essentially the same product category.
Think about it - for every iPad app, you could have a smaller version on the iPhone that does the same thing. For some apps, like drawing apps, the smaller version wouldn’t make quite as much sense given the fact that the iPhone has much less surface area to interact with. However, the larger display is still just something that makes certain tasks easier (as demonstrated by the larger number of people who prefer to watch movies and read books on tablets over phones). Some people use the iPad as a productivity device in place of a laptop, but arguably, they could do the same with an iPhone, it’s just harder to see, and people feel less efficient.
I don’t think there are three tiers of devices (phone, tablet, computer). I think there are only two, and that’s dictated not by form factor, but the OS the devices run. Traditional desktop class computers that run "proper" desktop OSes feature a much greater level of complexity that originates from the level of computing power they are allotted. Modern portable devices have more computing power than ever, but their functionality still originates from and has developed from the original product in its category. In this case, we could argue that is the first generation iPhone.
The iPad runs iOS and Android tablets still run Android. Those were OSes designed for phones and that’s just how they are. No matter how many tweaks and adaptations are made to improve usability at a larger screen size, the fundamental way the OSes work is the same and their functionality all comes from the original paradigm of a touchscreen phone OS.
A 7" netbook is still the same device as a 17" Alienware laptop, or even a desktop computer. You just buy the size that you can afford with the degree of portability and computing power you require.
I think that still applies in the space of smartphones and tablets, and eventually we’ll come upon a converged device for the average person. There will still be specialty devices for people with certain needs (the 11" MacBook Air for those who travel often and the 17" Alienware laptop for gamers), but the average laptop purchase is the 13.3" or 15.6" laptop that’s a balance of portability and power/usability.
Likewise, sub 4" phones are becoming much scarcer than they were a year or two ago, and 5"+ phones are becoming the norm. On the tablet side of things, the concept started with a 10" iPad, but 7-8" tablets like the Nexus 7 and iPad mini are becoming more popular than ever.
It’s only a matter of time before a single ~6" device becomes the norm and that the majority of people purchase.