I also don't think people with flagship android phones upgrade as often as iphone users. Everyone I personally know who owns the s3 has no interest in the s4, and will most likely use their current phone until it is not sufficient. Most people I know with iphones are always interested in the newest model if they can financially afford to upgrade every year.
The way i read it is that iphone lacks size and features whereas GS3 is feature rich enough (hard and soft) so as to make people fully happy and not needing any new handset soon