Think developing your apps to be compatible with those running old versions of iOS or Android. This is what massive adoption apps like WhatsApp is doing. WhatsApp newest version can be installed and run well in and old HTC Wildfire running Android 2.2. The HTC Wildfire has a very poor screen resolution and does not receive updates long time ago, but the app runs flawlessly.
Think on the developing countries with smartphones with low specs.
Think on the second hand market or give away phones like the phone you give to a member of your family or to a friend.
Think on your old phone as a back up for emergency, waiting on the drawer for when your new phone gets stolen, lost, falls on the water or its screen breaks. It would be nice to be able to use the latest apps on this old phone until the screen of the new one is replaced.
If the device still turns on, it still counts, at least for somebody.
Also allowing to reuse phones is a way of being friendly to the planet.
Or think in the different reasons people don't install the latest update on their phone. iPhone 4 can only run up to iOS 6, but is a perfectly functional phone.
IOS 7 brought a new look but some people just prefer the old user interface.
The iPhone 4S can run the latest iOS 9, but because performance problems lots of people will refuse to update.
Others may just lack the know-how to update their phone. Although easier now with the OTA (over the air) updates it still requires a generous amount of free space in the phone and it is never exempt of some risk. Apple updates are very difficult if not impossible to downgrade, so if you don't like the new system you can't go back.
