You guys have to consider how many have not ever had an iPhone before. Also consider those who are still using older phones or dumb phones. I think the high end Android phones these days are taking many of those potential iPhone consumers away. IMO, this is where Apple should have it's focus on if they want to continue it's iPhone growth. Growth may have already reached it's peak or will this year.
Now concerning existing users, I don't see Apple too worried about a small percentage of iPhone users leaving for something else. I think a higher percentage would leave if Apple played the same game that Samsung and HTC are playing. Most iPhone users appreciate Apple's iOS, quality, support, and the upgrades.
Apple is still in it's own bubble and is one of the most respected brands globally. Others like Samsung and HTC have no choice but to compete more vigorously.
BTW .... I just got a red GS3 w/64micoSD recently and still have my iP4(16gb). Just based on screen size and space alone, I prefer the GS3. I really don't have a preferred OS, but if I had to choose one on the same hardware ... iOS would most likely be my choice at the moment.