Thats because iOS is far different. You can download any app on an iPhone running 4.0.1 as you can with one running 5.1
Not sure what you're trying to say. A 4.0.1 iPhone cannot make use of 5.1 voice dictation APIs, for example.
If you mean that newer apps will fall back on using older code, that's true all over. Developers for both Android and iOS usually write code that's backwards compatible, in order to hit as many potential buyers as possible.
In both cases, an app is marked with the minimum OS level it requires, so the respective app store knows if the app should appear for a particular device.
Try downloading the newest version of Google Maps with navigation on an Android phone still running Froyo, good luck with that, aint happening.
I must have good luck. I just took an old Motorola Droid running Froyo from my drawer, and used the Market to update its Google Maps to the full latest ver 6.5 with navigation, 3D buildings, walking directions, compass nav mode, etc.
I could name another few dozen apps many people use that you can't get unless you at least have Gingerbread.
A few dozen? Sure, we'd love to see that list. Are they all 3D games or ?
So the point is, updates are pretty important for Android phones. Google wouldn't spend so much time creating several each year if they weren't.
Most Android OS updates nowadays are about speed and looks. They're not that important for adding basic functionality, because Android already has that and/or is so extensible through non-OS third party code.
I have devices running everything from Froyo to ICS, and haven't felt any great need for updates on the older ones, since they run all my favorite apps. (Netflix, Currents, eBay, USAToday, Pulse, Optimum, etc. and of course many popular games for my daughter.) I think Netflix was the last major phone app that I needed an upgrade for (and that was just to Froyo).