Only way to do that is to make it obsolete before its released. Playing with iOS 7 any new phone is gonna need to be similar spec'd as the 5 if not it will be a poor experience to begin with and never see iOS 8 a mere year later. Very unlike Apple.
So compared to a 4S it will likely have and as rumors point. A more expensive 4" screen, more expensive CPU, more expensive GPU, more ram, similar camera and a cheaper housing. AND sell for LESS then the 4S currently does?
How's that gonna work?
Another problem I see is I personally was dead set on the 5S. But I'm pretty rough on my phones. Now I'm considering this cheap iPhone. Apple effectively marketed to me to give them less money.
All I'm saying is I think there is more going on then we think. Maybe this cheap iPhone will replace the 5 and then 6 (or whatever it would be called) is gonna come out just brimming with new tech (bigger screen, NFC, larger internal storage, etc) therefore making the cheap iPhone a more reasonable solution to those that just want a basic smartphone. Just tossing out ideas.