As for "OpenGL is hard" - I think a major point here is that Apple has made it far less so. Because of their extreme curation of devices (re: only Apple produced devices), and the ease of software updates (well over 30% of phones are upgraded to the latest iOS version [1], with > 80% on a 5.x version [2]), iOS has continued to be a remarkable homogenous platform.
As far as Apple choosing a new graphics chip for the new device, it's certainly possible. With iOS, though, you can support a known list of devices and have 100% coverage rate. On Android you either have to whitelist phones (and therefore exclude users), or try to do feature detection at runtime to enable the correct codepaths.
[1] http://david-smith.org/blog/2012/05/11/ios-5-dot-1-1-upgrade... [2] http://david-smith.org/blog/2012/03/10/ios-5-dot-1-upgrade-s...