With iOS 9, the window will stretch back further than at any time in the iOS release history for compatible devices--back to 2011 for the iPhone 4s and iPad 2 (and 2012 for the iPod touch 5th generation). And some leaks about iOS 9 make it sound as though the release will be optimized for older devices, chewing fewer processor cycles for features they can't well support or support at all.
Malware developers try to pluck low-hanging, plump fruit, because they make their money or reap other rewards by selling their exploits or access to them to criminals and sometimes governments. Apple's choices do leave a significant number of users of older versions of iOS at risk, but simultaneously make them slim pickings compared to other options.
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