I don't think it works that way with tech products, because an outdated smartphone is considerably less functional than a cutting-edge one. I.e. there's a functional aspect to the desire that goes beyond fashion, or maybe fuses the two. Or are you implying that it will ever be fashionable to use an technologically outdated phone because the design is nice? (I could see it in some tiny contrarian subset of the population, but not as a general phenomenon.)
You're right about the internals, but this discussion was about external design cues. Even so, to the casual user (the majority), software use on iPhone 4 will not be "considerably less functional" than on the iPhone 4S so in this case an external cue would not signal any particularly important functional change.
