That's pretty much the problem with Apple. I agree, while it is indeed extremely beneficial for Apple to specialize in one display size, there are indeed trade offs, including lack of variation- Which is also listed as an disadvantage under Economics ! By specializing in simply one type of product, you are not providing consumers with much choice, and those who don't like such limits are compelled to switch.
The problem many people have with Apple now is why are they not making a bigger screen yet when the trend shows that a bigger screen is in great demand? At least make a second phone with a bigger screen right? Surprisingly, year after year Apple ignored this and continued to specialize in one display size. Which is why I believe 2014 is an extremely CRUCIAL year for Apple. The 5S is a great phone, but in terms of display size Apple is really stretching it this time round - it has consistently ignored market trend for nearly past 3-4 years ever since Samsung came into the market with a big screen.
The longer Apple took their time about the display size, the higher the demand for it and it will eventually KILL Apple. Yes, I repeat, KILL. Apple does have billions of dollars in the bank, but that does not mean it shouldn't fulfil its purpose of being a firm with a focus on profit maximization- Ideally Apple should experrience increasing profits year after year. In economics, its also said that firms who do not adjust to consumer demands will be driven out of the market. And that is what exactly happened to blackberry. Nokia nearly fell victim to it as well.
2014 is going to be a make or break year for Apple: release one phone with a permanent larger screen, or release a variation.
But I shall also add in that although consumers do want a larger screen, it does not necessary mean they MUST have a larger screen. Some want a larger screen but is actually fine with the current screen size. There is a fine line between a MUST and a WANT. But i do believe there is still no harm in releasing an iPhone with a larger screen.