I would think that Apple would do enough polling or whatever to determine if sales would justify making a 4.5" or 5" screen. If enough profit can be made, why not make a larger screen? I think that as long as the larger screened phones continue to sell well, companies will put these out because enough consumers do want them.
I hear some posters saying "iPhone users do/don't want...". However, there are so many more iPhone users out there than the vast majority of us will come into contact with that it's hard to say what the majority of iPhone users want. My wife who is small/petite and has small hands has used everything from a 2.8"-4.5" device and the 4.5" device is her favorite.
Samsung has done well with their 4.6"-5.5"+ screens despite the naysayers and those concerned enough with what people are using to make fun of them for what they look like holding a large device to the side of their face. I guess there are enough of us (Note users) that just don't care what others think about us.
Where does it end though? Apple already sells a 4 inch phone, a 7 inch tablet and a 10 inch tablet. Plenty for most consumers. You really think they can make a device for every person's own want? Sure! Lets make a 4 inch, 5 inch, 6 inch iPhone and hey while we are at it, lets make a 9 inch iPad too just in case the mini is too small and the 10 inch is too big