Even when an iOS device can run the latest version, it doesn't mean it runs it smoothly…
My iPad 2 (just over two years old) is a real pain with iOS7, virtually every time I switch apps the app has to restart from scratch
And you can still buy an iPad 2 brand-new, likely for the same price you paid...
Yes, this is true, especially in the past (up to the iPhone 4/ iPad era). Obviously hardware dependent features aren't there (eg siri when new, iOS7 camera, M7, etc) either.
Still, I have zero issues with iOS 7 on my iPad 2 and my parents are happy on their 4S.
Some of my relatives who took my iPad 1s when I bought new Ipads for my kids are stuck on iOS 5.1.1. Those iPads are from September 2010, but stopped receiving any updates for system apps as of May 2012.
So yes, granular, loosely coupled updates are better than "rebuild the whole tightly coupled world and ship a firmware-diff" updates.
In particular, if an app is not tied to specific kernel features on Android, changes are it can have along life, because Android OS level services like Google Play Services can be independently updated as well.