Hardware differences get in the way
This is about more than apps, of course. But it's mostly about apps, because there are hardware differences -- some natural, some vendor-imposed -- that mean a smartphone or tablet today can't yet scale up to become a PC through, say, Bluetooth and Wi-Fi Direct peripherals. (Hewlett-Packard says it'll try to deliver that on Windows Phone; maybe that'll fare better than previous attempts in the Android world.)
But I bet that in the next four to five years we'll see many of the hardware barriers disappear. Many of the barriers are artificial, such as Apple not wanting iPads to replace MacBooks but instead keep supplementing them.
I don't believe we'll end up with only one device, but that the overlap among them will be greater -- and the ability for a device to scale up or down will also be greater, so you have more flexibility when not in your regular environments.
Systems management points the way to unity
Another domain where the postmobile world is emerging is in systems management. Four years ago, Apple largely unified the management APIs between iOS and OS X.
Microsoft is doing the same now with Windows 10 and Windows Phone, in a way that also would allow PCs, Macs, iOS devices, Android devices, and Windows phones to be managed centrally using the same core API-driven technologies. Basically, they're all devices, so manage them all that way.
The bottom line: Stop thinking of the technology world as a binary mobile/desktop dichotomy. It's becoming one world where different devices do the computing you're providing via apps, services, APIs, and -- one day -- peripherals. The sooner you accept that, the sooner your wares will be used in all those devices.
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