From the iPhone 3GS release in mid 2009, through to iOS 6, hundreds of features have been added. Of these hundreds, only three aren't on the iPhone 3GS - Facetime, Siri, and the new mapping system.
From the iPhone 4 release in mid 2010, through to iOS 6, hundreds of features have been added. Of these hundreds, only two aren't on the iPhone 4 - Siri and the new mapping system.
This includes all of the security updates, and all of the hundreds of thousands of apps that require the latest iOS version to run (upgraded APIs and such).
In contrast, 93% of Android devices cannot run Google's own Chrome browser. This is the price of fragmentation.