Great point, but sadly that's always how it will be. Just like if the iPhone 5 ended up having the same size screen as the SIII these same people would probably be all over it.
Most likely I'd buy the iphone if they gave it a much bigger screen (though maybe I'd go with the 4s depending on what else they changed about the phone). And I'd be whining about it the whole time.
And then one of two things would happen:
1. I'd realize I was wrong and had to eat my words. And count for the ones people will claim will all the sudden like the phone once Apple makes it (because most likely I'd buy it anyways cause of other considerations like the fact I already have a lot of apps and stuff and am pretty tied into the eco system and don't really want to change if I don't have to, Android really hasn't offered anything I'm all that jealous of enough to want to deal with breaking out of the eco system). But thing here is I would have been dragged into it kicking and screaming and after trying it have changed my mind (people do that you know).
2. I'd realize i was right and bitch and whine about it some more and wish they never changed the screen size. And realize I don't have much choice caues it seems the Androids with the good features all have big screens too and the cheapy phones that probably suck will be the only ones left with a smaller screen if the iPhone also gets a big screen. Therefore no reason to change anyways. Just be annoyed they changed it.
But really, if they do change the screen size, where do you expect the people who liked the smaller screen to go? All the "prize" android phones that will have the nice features all have big screens (They race to outdo each other on how big the screen is), I think even the Windows phones are the same. So, it's not like if you want the smaller screen if Apple changes that you have much anywhere to go (Apple seems to be the only premium smartphone maker that has not done the race to see who can make the biggest screen). If nothing else is ruined and they liked the iPhone otherwise, they might as well stay with the iPhone.