If an iPhone can no longer get a major version upgrade (e.g. a N.x upgrade not an N.0x upgrade), it also can't get any of its apps updated.

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.