Then they should stop using their iPhone and buy a phone with a bigger screen? The vast majority of Android phones with a bigger screen are way cheaper than iPhone anyway, so it's not like money is preventing them from switching.Errr..I know plenty of people that have made the switch for this exact reason.
I also know iPhone owners who complain non stop about the tiny size of the screen.
Personally, I wish I could keep my 3.5" iPhone 4 forever because it's the perfect size for me. If Apple ever releases a 5" iPhone I certainly hope they'll keep the 4" models for people who want a device that fits in their pocket.