This doesn't really hold up because it's not your typical update, like e.g. updating from Windows 7 to Windows 8, where you have to actively take steps to purchase a new copy and install it.
Alerts/badges about this update are pushed to your phone for free, with little to no effort to make clear to the user that it's a significant update that apparently requires research.
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What's in question is if Apple is to some extent responsible. For me, these three things contribute to their responsibility:
1) Selling 3 year old iPhone models (during summer 2013)
2) Allowing these models to update to iOS 7
3) Pushing notifications about this update to the device and making it installable in a few easy taps