Why do you care what others buy? I've never felt fear for another's choice of phone they bought.
Use what you like and move on. No need to fear others may buy another manufacturer's phone.
Why care what others buy? Isn't it obvious? I think there's a "love" for Apple, inspired by art, design, excellent customer care, Steve (of course) and many other factors.
Apple has managed to do what most companies and marketing teams only dream of - They have created fans out of customers.
As pointed out earlier, the "Use what you like and move on" attitude has not served Blackberry particularly well. I suspect that if Apple offered a 4.5" screen alongside a 4" screen this year, the larger model would sell the most by far.
Maybe Apple's own marketing is to blame for setting expectations too high in the past. Many have become accustomed to being surprised and delighted with their new Apple product, and when a company they admire takes 2+ years to produce size variants that people clearly would like to be able to choose from, it's disappointing to some.
If you're an Apple fan and want continued success and growth in market share for the company, then "fearing" defections (which is already happening, in my opinion) seems a reasonable choice of word to me. I want lots of people using iPhones, so developers continue to build for the platform and apps that rely on crowdsourcing (like live traffic) remain effective and functional. When people start leaving for other platforms, especially over something that is so easily addressed, I believe it will eventually have a downside in some ways for those who stay.