I think we're overlooking a crucial factor.
The Snapdragon 820 is more powerful that the processors inside a large chunk of laptops sitting on shop shelves today (think AMD Fusion series, Intel Atom).
With Cortex A72, it's possible to have a phone that also functions as a mobile computer CPU, which you just plug into a I/O terminal (a kind of dumb workstation) of the future.
My strong belief is that by 2017, a lot of people will stop having dedicated computers and their phone will be their computer. When on the move, they'll use the built in screen and touch typing, and at other places, they'll hook it up to a 'full size' display and input devices.
You'll also see dumb laptop-type cradles which will come to life when you plug in your phone. And in such a scenario, I'm sure a modular design becomes more plausible compared to the current scenario. I mean, if you're using your phone as you do now, you probably don't care about how powerful your CPU can be next year if you buy the latest one.
But we're moving towards phone-PC convergence by 2017.
I wonder why companies keep trying the modular phone approach. People don't want it, and I think that it's a bad idea.
I get that in theory enthusiasts or whatever would like to have a modular phone.
new snapdragon? new wifi standard? radio? more RAM? better camera? whatever - just add a module.
In real life, most people don't care. They don't know how much RAM their phone has or how good the CPU is. A lot of people think that the iPhone 6s is "barely an upgrade" vs the iPhone 6. It also adds a lot of complexity. Which camera module should I buy? what's the difference between these two 2gb RAM modules? Do we now need to handle drivers for different modules? are things hot swappable? Can damage be done more easily if you drop the phone and a module loosens? etc. etc.
We'll see I guess, but I believe modular phones are born to fail.