It's pretty important to user satisfaction that Joe can say to Sally "You have an iPhone, right? Check out what they can do now!" And Sally be able to say "Mine does that too? Awesome!"
Sally won't be able to do Turn by turn if she has a 3GS or iPhone 4, whereas even out of date Android's on 1.6 can.
"Sure, with iOS there are updates and features that don't get patched into older devices. But those situations are vanishingly few compared to Android."
But the "vanishingly few" are often the most significant features of a particular years iOS upgrade, whereas Google's 1st party apps are for the most part decoupled with the major differences between 4.X and 2.X being UI.
The situation is much more nuanced than you state.
Sally won't be able to do Turn by turn if she has a 3GS or iPhone 4, whereas even out of date Android's on 1.6 can.