Another data point on upgrade cycle: I dropped my Asus Transformer and cracked the screen recently. While trying to make a repair-or-replace decision, I looked around for options with a similar rigid hinge detachable keyboard and came up unimpressed. All far too expensive; much easier to spend £20 and a couple of (slightly nervewracking and fiddly) hours replacing the touchscreen.

I suspect a large driver of phone replacement is more frequent accidents, loss, and theft.